Transitional event (4,2,7)
I believe the answer is:
rite of passage
'transitional event' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rite of passage that I've seen before include "Ceremony marking stage of life" , "The Wedding, perhaps?" , "Formative experience" , "Significant event during a transitional period in life" , "Very important life event" .)