Sunday nights works best for me.
My personal recommendation, though, would be to have a few weekly chats at different times.
This would allow all of our friends from all over the world to join in at a time convenient to their time zones rather than having one shot - take it or leave it.
Of course, anybody would be welcome to participate in any of the chates, having several choices would allow for better participation over all.
That is just my recommendation for whatever it's worth.