Where does “lễ Tạ Ơn” come from?

lễ Tạ Ơn (Vietnamese) comes from Vietnamese lễ, from Proto-Vietic k-lɛː.

Ancestry of “lễ Tạ Ơn”, step by step

Step	Language	Word	Meaning
1	Vietnamese	lễ	bribe; offering; festival, festive day, holiday;...
2	Proto-Vietic	k-lɛː	—