Christian festival (6)
'christian festival' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for easter that I've seen before include "Christian celebration of the Resurrection" , "Snug retreat" , "Annual religious festival" , "Resurrection festival" , "Christian celebration at the end of Lent" .)
(Other definitions for mardi gras that I've seen before include "Time for carnival" , "festive occasion" , "Carnival (day)" , "pre-Lent festival" , "Shrove Tuesday festival" .)