Arrival one day before opening (6)
I believe the answer is:
advent
'arrival' is the definition.
(advent is a kind of arrival)
'one day before opening' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'day' becomes 'd' (abbreviation for day).
'before' is a charade indicator (letters next to each other).
'opening' becomes 'vent' (vent is a kind of opening).
'a'+'d'+'vent'='ADVENT'
(Other definitions for advent that I've seen before include "Christmas shopping time in" , "introduction" , "The coming of a religous period?" , "approach" , "Season preceding Christmas" .)