Day of Atonement (3,6)
I believe the answer is:
yom kippur
'day of atonement' is the definition.
(synonyms)
This is all the clue.
(Other definitions for yom kippur that I've seen before include "Annual solemn Jewish Day of Atonement" , "Day of atonement, Jewish festival" , "Holiest fast day of the Jewish calendar" , "Jewish holy day" , "Religious fast" .)