First day of Lent (3,9)
I believe the answer is:
ash wednesday
'first day of lent' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ash wednesday that I've seen before include "Date in church calendar" , "Time to start doing without" , "First day of Lent" , "Start of Lent" , "Beginning of Lent" .)