SmileBASIC, 73 71 67 bytes

?"Christmas";

@L?" Eve"*(D!=P);

P=D

DTREAD OUT,M,D

IF M/D-.48GOTO@L

The program prints "Christmas", then prints " Eve" every time a day passes, until it is December 25th. (12/25 = 0.48)

May take up to a year to run.

Excel formula, 62 bytes

="Christmas"&REPT(" Eve",DATE(YEAR(NOW()+6),12,25)-TODAY())

R, 112 106 72 bytes

Via @digEmAll and @J.Doe

x=Sys.Date()-1;cat('Christmas');while(!grepl('12-25',x<-x+1))cat(' Eve')

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My original answer was prior to the clarification that the code was to take the date on which the code is run as input. It could be modified as above to save many bytes but I won't bother.

function(x,z=as.Date(paste0(strtoi(format(x,"%Y"))+0:1,"-12-25"))-x)cat("Christmas",rep("Eve",z[z>=0][1]))

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Explanation: everyone's at church so I have time to do this. Extract the year, coerce to integer. Make vector of that year's Xmas and the next year's Xmas and subtract the input date to get a vector of two differences between the input date and those two Xmases.

Pick the non-negative one and cat "Christmas" with that many "Eves".

Perl 6, 61 47 bytes

say 'Christmas'~' Eve'x(Date.today...^{.month==12&&.day==25})

say 'Christmas'~' Eve'x(Date.today...^/12-25/)

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-14 bytes (!) thanks to Jo King

Date.today ...^ /12-25/ is the sequence of dates starting today and ending the day before Christmas. (The regular expression /12-25/ is matched against the string representation of the dates.) The string " Eve" is replicated a number of times equal to the length of that sequence, and is output after the string "Christmas".

Windows PowerShell, 67 64 63 bytes

for(;1225-'{0:Md}'-f(date|% *ys $i)){$i++}'Christmas'+' eve'*$i

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Managed to shave off 3 bytes 4 bytes (thanks Cows quack) by using the -format operator instead of .ToString(), and then subtracting the date string from the numerical value 1225 instead of doing a comparison with -ne. The resulting integer will be interpreted as a boolean for the conditional where 0 (which will happen on Christmas) is interpreted as False (don't enter the loop), and any other value is interpreted as True (enter the loop).

Since the integer is on the left now, the date string will be converted to the integer and math will be done, as opposed to the previous version where the 1225 integer was converted to string for the comparison.

Original Version

Windows PowerShell, 67 bytes

for(;(date|% *ys $i|% tost* Md)-ne1225){$i++};'Christmas'+' eve'*$i

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Using a for loop as a while loop basically, because it's shorter. In the loop condition we check the current date (date, a shortened form of Get-Date), piped to ForEach-Object's alias %, using the form that can invoke a method by wildcarded name; in this case the method is AddDays() on the DateTime object, and the value we give it is $i.

This gets piped to ForEach-Object again to invoke the ToString() method, with format string Md (month, then day, minimal digits since we don't care for what comes next). This string is then tested to see if it's not equal -ne to the number 1225, which will be converted to a string for the comparison, saving me the quotes.

This is why it doesn't matter that the months and days are single digits, it will never be ambiguous because there's no other day of the year that would stringify to 1225.

The loop continues until the string is 1225. At the beginning of the program, $i will be zero so it will be comparing today's date, and the loop will never execute, but for any other day $i gets incremented in the loop body, so that we will have a count of how many days until the next Christmas, automatically accounting for leap years and whether or not Christmas passed this year.

After the loop we just output the string Christmas concatenated with the result of multiplying the string eve times the value of $i (which, on Christmas day, will be 0, resulting in no eves).

T-SQL, 92 bytes

SELECT 'Christmas'+REPLICATE(' Eve',DATEDIFF(DAY,GETDATE(),STR(YEAR(GETDATE()+6))+'-12-25'))

C# (Visual C# Interactive Compiler), 89 bytes

Write("Christmas");for(var t=DateTime.Now;$"{t:Md}"!="1225";t=t.AddDays(1))Write(" Eve");

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-3 bytes thanks to @JeppeStigNielsen!

My strategy is pretty straightforward:

1. Initialize a loop variable t to the current date


2. Print Eve if t is not Christmas


3. Add a day to t and repeat

I tried some fancier things, but this way required the fewest bytes.

APL (Dyalog Unicode), 76 63 bytes^SBCS

Full program. Assumes ⎕IO←0 (zero-indexing).

⎕CY'dfns'

'Christmas',' Eve'⍴⍨4×12 25⍳⍨⍉2↑1↓⍉date(⍳366)+days⎕TS

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⎕CY'dfns' copy in the dfns library

⎕TS current time stamp as [year,month,day,hour,min,sec,ms]
days^[c] find the number of days^[n] since 1899-12-31 00:00:00.000
(⍳366) add the first 366 integers (0…365) to that
date^[c] find the dates^[n] that correspond to those numbers (366×7 table; one column per unit)
⍉ transpose (7×366 table; one row per unit)
1↓ drop one row (the years)
2↑ take the first two rows (months and days)
12 25⍳⍨ find the index of the first Christmas
4× multiply that by four
' Eve'⍴⍨ use that to reshape the character list
'Christmas ', append that to this

^[c] code of that function
^[n] notes for that function

PHP, 61 bytes

Christmas<?for($t=time();date(md,$t+=86400)-1226;)echo" Eve";

Run with -n or try it online.

Ruby, 80 bytes

require'date'

t=Date.today

puts'Christmas'+' Eve'*(Date.new((t+6).year,12,25)-t)

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Thanks to tsh for his idea

JavaScript, 135 131 121 92 bytes

My first (naïve) solution (135b):

t=new Date();n=new Date();n.setMonth(11);n.setDate(25);'Christmas'+' Eve'.repeat((n>=t?n-t:(n.setFullYear(n.getFullYear()+1)-t))/864e5)

It sets 2 dates: now and Xmas of this year. If the latter hasn't passed yet, it just diffs them, if it has passed, diffs to next year's Xmas. Uses either diffs for the number of repeats.

(Trying to) Think Outside the Box (131b):

i=0;f=_=>{t=new Date();if(t.getMonth()!=11||t.getDate()!=25){i++;setTimeout(f,864e5)}else{alert('Christmas'+' Eve'.repeat(i))}};f()

The challange specifies WHICH output is required when running the program on a given day, but doesn't specify WHEN to return it...

This will just 'sleep' for a day, increment a counter by 1, and repeat till it's Xmas in order to give the output.

Since JavaScript doesn't guarantee the 'sleep' time, the actual result might be off.

It is also ugly for using the alert function, which means wer'e actually not dealing with pure JavaScript, but with browser APIs as well (we can use console.log at the cost of 6 extra bytes).

A better approach (121b):

t=new Date();i=0;while(t.getMonth()!=11||t.getDate()!=25){t=new Date(t.valueOf()+864e5);i++};'Christmas'+' Eve'.repeat(i)

Starting from today, increment the date by a day until it's Xmas, then use that loop's counter for the number of repeats required.

Improving (including going through a minifier and using 12Me21's trick to shave extra 5b) (92b):

for(s='Christmas',t=new Date;t.getMonth()/t.getDate()-.44;)t=new Date(t*1+864e5),s+=' Eve';s

Bash, 68 65 bytes

seq 0 366|sed 's/.*/date -d&day/e;1iChristmas

/c 25/Q;cEve'|xargs

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BSD date should be able to save a byte with something like date -v+Ad (can't test it), however, BSD sed would add more bytes to i and c, requiring them to have a <newline>.

seq 0 366 create a stream of integers from 0 to 366

|sed perform the following sed code over each line of stream input

• 
  s substitute


• 
  
  • 
    .* the pattern space
  
  


• 
  
  • 
    date -d&day with this string, with the match filling the place of &
    
  
  


• 
  
  • 
    e replace the pattern space with itself evaluated as bash, which computes the date & days from today in the default format of Wed Dec 26 18:22:33 UTC 2018
    
  
  


• 
  1 on the first line of input


• 
  
  • 
    i insert
  
  


• 
  
  • 
    
    • 
      Christmas this string above the line, so being on top of the output
    
    
  
  


• 
  /c 25/ if the current line has a c 25 in it, meaning it's Dec 25


• 
  
  • 
    Q quit the program without printing the pattern space, abruptly stopping any more lines from being read
  
  


• 
  c (otherwise) change the current line to


• 
  
  • Eve
  
  

|xargs and convert newlines to spaces

VBA (Excel), 108 bytes

Copy in a blank module. Prints to the Immediate window:

Sub X:s="Christmas":d=Now:For t=1 To (DateSerial(Year(d+6),12,25)-d):s=s &" Eve":Next:Debug.Print s:End Sub

Note: Using : instead of line breaks saves two bytes per line.

Notice that the VBA editor will insert additional spaces between keywords, operators, etc... and parenthesis after the Sub definition, but if you copy and paste this code it will work (I couldn't get rid of that space before the &).

Not bad for VBA (for once).

Python 2, 111 103 bytes

from datetime import*

d=date.today()

print"Christmas"+" Eve"*(date((d+timedelta(6)).year,12,25)-d).days

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Update inspired by Richard Crossley's answer.

Explanation:

from datetime import*

# get today as a date, so we don't have to worry about rounding errors due to time

d=date.today()

                              # get the year of the Christmas to compare to

                              # if the current date is after this year's Christmas, the 6 day offset will give the next year

                              # otherwise, returns this year

                              (d+timedelta(6)).year

                         # next Christmas minus the current date

                         date(.....................,12,25)-d

# Christmas, plus (number of days until next Christmas) " Eve"s

print"Christmas"+" Eve"*(...................................).days

Bash +GNU date, 72 73 bytes

for((d=0;1`date +%d%m -d$dday`-12512;d++));{ x+= Eve;};echo Christmas$x

• one byte saved replacing != with -
  


• another removing extra space


• fix -3 bytes d=0, because date -dday is date+1 and doesn't work on 25/12

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Python 2, 128 bytes / Python 3, 130 bytes

of course, two less bytes with Python 2

from datetime import date as D

T=D.today()

Y=T.year

a=(D(Y,12,25)-T).days

print("Christmas"+" Eve"*[a,(D(Y+1,12,25)-T).days][a<0])

C (gcc), 157 bytes

I thought that I would be able to avoid including time.h but that just gave segment faults.

#include <time.h>

*t,u;f(){time(&u);t=localtime(&u);t[5]+=t[4]>10&t[3]>25;t[4]=11;t[3]=25;u-=mktime(t);printf("Christmas");for(u/=86400;u++;printf(" Eve"));}

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Groovy, 66 bytes

d=as Date

print'Christmas'+' Eve'*(new Date((d+6).year,11,25)-d)

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Courtesy of @ASCII-only

MySQL, 102 bytes

pretty much the same as Neil´s T-SQL answer. There seems to be no shorter way in SQL.

select concat("Christmas",repeat(" Eve",datediff(concat(year(now()+interval 6 day),"-12-25"),now())));

Try it online.

Scala, 140 bytes

import org.joda.time._

var s="Christmas"

var d=DateTime.now

while(d!=d.withDate(d.year().get(),12,25)){d=d.plusDays(1);s+=" Eve"};println(s)

Does not run in TIO since it requires Joda-Time library.

Java 8, 161 146 Bytes

void t(){LocalDate a=LocalDate.now();

String b="Christmas";

while(!a.toString().endsWith("12-25")){b+=" Eve";a=a.plusDays(1);}

System.out.print(b)}}

Python 3, 106 Bytes

from datetime import*

d=date.today()

print("Christmas"+" Eve"*(date((d+timedelta(6)).year,12,25)-d).days)

Scala, 116 113 bytes

var d=new java.util.Date

print("Christmas")

while(!(""+d).contains("c 25")){print(" Eve");d.setDate(d.getDate+1)}

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Where c 25 is short for Dec 25.

MATLAB, 91 bytes

n=datetime

x=datetime(year(n+6),12,25)

s='Christmas'

while days(x-n)>=1 n=n+1 s=[s,' Eve'] end

MATLAB Non-looper, 100 bytes

x=datenum(datetime(floor((now+5)/365.2425),12,25))

d=x-now

['Christmas' repmat(' Eve',1,min(d(d>=0)))]

JavaScript 86 bytes

Using REPL it would be

for(c='Christmas',d=new Date;(d+'').indexOf('c 25')<0;d=new Date(+d+864e5))c+=' Eve';c

C# (Visual C# Interactive Compiler), 141 bytes

var g=DateTime.Now;Write("Christmas"+string.Concat(Enumerable.Repeat(" Eve",(new DateTime(g.Year+(g.Day>25&g.Month>11?1:0),12,25)-g).Days)));

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Red, 89 86 84 78 76 bytes

-10 bytes thanks to ASCII-only!

does[a: now prin"Christmas"while[a/3 * 31 + a/4 <> 397][prin" Eve"a: a + 1]]

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Perl 5, 68 bytes

print"Christmas";print" Eve"while localtime($i++*86400+time)!~/c 25/

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Where c 25 is short for Dec 25.

perl -E, 25 bytes

perl -E'say"Christmas"," Eve"x365'

This will print the required answer -- and then some extra characters. But since that wasn't explicitly forbidden, I'm just going to bend the rules. Heavily.

PHP, 84 bytes

Probably doesn't work that well.

$d=intval(date("z"));echo("Christmas ".str_repeat("Eve ",(358-$d)<0?724-$d:358-$d));