I can't see a thread for this - though I'm certain it is today. I don't think it's like Christmas or Easter in terms of not being celebrated by reformed Christians.
So I think it is OK to say
Happy Thanksgiving Day to all our US brothers and sisters
(if I've dropped a clanger I'm sure someone will tell me)
So I think it is OK to say
Happy Thanksgiving Day to all our US brothers and sisters
(if I've dropped a clanger I'm sure someone will tell me)