Solemn observance
(Other definitions for ceremonial that I've seen before include "Official" , "Ritual(istic)" , "Ritual procedure" , "Purely formal" .)
'solemn observance' is the definition.
(eg a religious rite)
This is all the clue.
(Other definitions for rite that I've seen before include "form of service" , "Solemn ceremony" , "Ceremony - ritual" , "Ceremony, liturgy" , "something like mass" .)