SmileBASIC, 73 71 67 bytes

?"Christmas";

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

O=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.

R, 112 106 bytes

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".

Excel formula, 62 bytes

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

Perl 6, 61 bytes

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

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Date.today ...^ { .month == 12 && .day == 25 } is the sequence of dates starting at today and ending the day before Christmas. The string " Eve" is replicated a number of times equal to the length of that sequence, and is output after the string "Christmas".

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

C# (Visual C# Interactive Compiler), 92 bytes

Write("Christmas");for(var t=DateTime.Now;t.Month<12|t.Day!=25;t=t.AddDays(1))Write(" Eve");

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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.

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

T-SQL, 92 bytes

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

PHP, 61 bytes

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

Run with -n or try it online.

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())));

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Python 2, 129 bytes / Python 3, 130 bytes

of course, one less byte 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])

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));

Groovy, 156 bytes

import java.time.LocalDate as D

d=D.now()

c=D.of(d.year,12,25)

'Chistmas'+' Eve'*java.time.temporal.ChronoUnit.DAYS.between(d,d>c?c.withYear(d.year+1):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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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

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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Red, 89, 86 84 bytes

-2 bytes thanks to ASCII-only!

does[a: now/date prin"Christmas"while[(a/3 <> 12)or(a/4 <> 25)][prin" Eve"a: a + 1]]

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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).