Oops, it doesn't look like 'sundae sundae day' is a national day..

Other days

offer

Offer Day
gum

Gum Day
youth

Youth Day
carpenters

Carpenters Day
nae nae

Nae Nae Day
self care

Self Care Day
ruby

Ruby Day
slime

Slime Day
chocolate milkshake

Chocolate Milkshake Day
artists

Artists Day