We don't have synonyms for tag day yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are rag day and tax day.
(n) :
(US, Bermuda, Canada) A fundraising day on which contributions to charity or funds are solicited on the street, or as donations to a charitable event, and tags given to contributors to wear as evidence of their contributions.
(n) :
(US, Bermuda, Canada) A fundraising day on which contributions to charity or funds are solicited on the street, or as donations to a charitable event, and tags given to contributors to wear as evidence of their contributions.