Festival held two days after Quinquagesima Sunday (5,4)

I believe the answer is:

mardi gras

(Other definitions for mardi gras that I've seen before include "Shrove Tuesday celebration" , "The lead up to fast" , "Carnival (day)" , "Day for 9 acrosses" , "Revelry celebrating Shrove Tuesday" .)

I've seen this clue in The Sunday Times and The Times.

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