The most common and most likely answer to this crossword clue is the 6 plus 3 letter word SAINTS DAY
I've seen this crossword clue in The Daily Telegraph
S
A
I
N
T
S
D
A
Y
More clues leading to the result SAINTS DAY
Buddy explains!

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