The most common and most likely answer to this crossword clue is the 10 plus 3 letter word RED-LETTER DAY
I've seen this crossword clue in The Guardian
R
E
D
-
L
E
T
T
E
R
D
A
Y
More clues leading to the result RED-LETTER DAY
Buddy explains!
The straight part
We know that "special occasion/special occasion" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case special occasion/special occasion = red-letter 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.