The UK just released a set of stamps called “Curious Customs” which celebrates British annual celebrations and folk customs, including a few that are mumming-related:
The Padstow ‘Obby ‘Oss
The Abbots Bromley Horn Dance
And I suppose that a giant Halloween celebration of any kind is at least mumming-adjacent
Masks & Monsters 
that is so freakin’ cool! i recently did get a sheet of monster stamps here.
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