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".

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".

Excel formula, 135 bytes

="Christmas"&REPT(" Eve",IF(TODAY()>=DATE(YEAR(TODAY()),12,25),DATE(YEAR(TODAY())+1,12,25)-TODAY(),DATE(YEAR(TODAY()),12,25)-TODAY()))

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.

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])

Red, 89 bytes

does[a: now/date prin"Christmas"while["25-Dec"<> take/part mold a 6][prin" Eve"a: a + 1]]

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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

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)

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.

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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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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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));