Mark an occasion with festivities (9)
I believe the answer is:
celebrate
'mark an 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 "Rejoice - observe - perform" , "Mark an occasion with a social gathering" , "Perform with appropriate rites" , "Have a party to mark a happy event" , "Be festive" .)