The most common and most likely answer to this crossword clue is the 7 plus 5 letter word MOVABLE FEAST
I've seen this crossword clue in The Times
M
O
V
A
B
L
E
F
E
A
S
T
More clues leading to the result MOVABLE FEAST
Buddy explains!
The straight part
We know that "E.g. Pancake Day" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case e.g. pancake day = movable feast.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.