The name of the holiday is 'Thanksgiving'. If you're just writing a note to someone it's not necessary. Other uses of the term are optional, for example if the teacher writes the message on the board...
Halloweenas BeunasFeliz noche de brujas
le happy alloween
The Romanian language doesn't have it's own dependent word for Halloween, so the translation, which involves the English word, is Halloween fericit.
Happy Halloween in Welsh is: Calan Gaeaf Hapus