Mark an important occasion with festivities (9)
I believe the answer is:
celebrate
'mark an important occasion with festivities' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for celebrate that I've seen before include "Throw a party" , "Make merry" , "have a good time" , "Have a party in honour of" , "Stage a special festivity" .)