The most common and most likely answer to this crossword clue is the 8 letter word PASSOVER
I've seen this crossword clue in The Daily Telegraph
P
A
S
S
O
V
E
R
More clues leading to the result PASSOVER
- Festival finished after meeting between father and sons
- Cleric, very big fool, set up religious festival
- Miss religious festival
- Festival admission ticket a thing of the past
- Don’t mention a festival in spring
- Elements of football and cricket used in religious festival
- An event Jews won’t forget or ignore
- . Skip the feast
- Go to see festival
Buddy explains!

The straight part
We know that "religious festival" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case religious festival = passover.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.