The most common and most likely answer to this crossword clue is the 2 plus 7 letter word ID AL-FITR
I
D
A
L
-
F
I
T
R
Buddy explains!
The straight part
We know that "a festival day" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a festival day = id al-fitr.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.