Celebration fixed around mid-September (5)
I believe the answer is:
feast
'celebration' is the definition.
(feast is a kind of celebration)
'fixed around mid-september' is the wordplay.
'fixed' becomes 'fast' ('fast' can be a synonym of 'fixed').
'around' indicates putting letters inside.
'mid' means to look at the middle letters.
'september' becomes 'sep' ('Sep' can be a synonym of 'september').
The central letter of 'sep' is 'e'.
'fast' going around 'e' is 'FEAST'.
(Other definitions for feast that I've seen before include "Huge nosh-up" , "Extravagant meal" , "entertainment" , "'Enough is as good as a .....'" , "Saint's day, for example" .)