From fest, from GermanFest ("festival"), abstracted from Volksfest, etc.
Full definition of -fest
Suffix
A festival, a fest; used in names of events.summer → Summerfest
(informal) Appended to a noun to denote a thing, especially an event or artistic work, characterized by the noun's referent.snooze → snoozefestslug → slugfestaction → actionfestjoke → jokefest