The most common and most likely answer to this crossword clue is the 6 plus 3 letter word EASTER DAY
I've seen this crossword clue in The Guardian
E
A
S
T
E
R
D
A
Y
More clues leading to the result EASTER DAY
Buddy explains!
The straight part
We know that "Eve of historic event in 1916/for which date’s varying with year" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case eve of historic event in 1916/for which date’s varying with year = easter day.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.