Coming a day before the opening (6)
I believe the answer is:
advent
'coming' is the definition.
(advent is the arrival or coming of something)
'a day before the opening' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'before' is a charade indicator (letters next to each other).
'the opening' becomes 'vent' (a vent is an opening or outlet).
'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" .)