The most common and most likely answer to this crossword clue is the 2 plus 7 plus 3 letter word ST DAVID'S DAY
I've seen this crossword clue in The Times
S
T
D
A
V
I
D
'
S
D
A
Y
Buddy explains!
The straight part
We know that "opportunity for celebration" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case opportunity for celebration = st david's 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.