The most common and most likely answer to this crossword clue is the 5 plus 2 plus 5 letter word FEAST OF WEEKS
I've seen this crossword clue in The Times
F
E
A
S
T
O
F
W
E
E
K
S
Buddy explains!
The straight part
We know that "religious celebration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case religious celebration = feast of weeks.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.