In this sentence, 'traditionally' refers to the main verb 'is', not to the infinitive 'to get together'. Somehow, 'to get together traditionally' makes no sense, although in other structures 'traditionally' could come before 'get together'
'Families traditionally get together on Thanksgiving Day.'
'Thanksgiving is the day when families traditionally get together.'
I'm looking forward to hearing a better explanation than mine!
As Hermione said, it follows is ("the be verb") because is is the verb it refers to. "To get together traditionally" would mean that there is a traditional way to get together, and that this is the way they do it.