Coming a day before the opening (6)
I believe the answer is:
advent
'coming' is the definition.
('advent' can be a synonym of 'coming')
'a day before the opening' is the wordplay.
'day' becomes 'd'.
'before' is a charade indicator (letters next to each other).
'the opening' becomes 'vent' (vent is a kind of opening**).
'a'+'d'+'vent'='ADVENT'
(Other definitions for advent that I've seen before include "Period in church calendar" , "Period in the Christian calendar" , "Coming (of Christmas)" , "Second Coming" , "Time approaching Christmas" .)