Day before National Trust’s planned occasion (5)
I believe the answer is:
event
'planned occasion' is the definition.
(a scheduled event)
'day before national trust's' is the wordplay.
'day before' becomes 'eve' (I've seen this in another clue).
'national trust' becomes 'nt'.
'eve'+'nt'='EVENT'
(Other definitions for event that I've seen before include "Item in sports programme" , "High jump, say" , "Incident - final outcome" , "major incident" , "Something happening" .)