I, personally, would prefer weeknight evenings, but I think that it is going to vary a lot according to the people who join your group. I'd start with whatever is convenient for you and then poll people who show up as to what they would prefer for future meetings.

I've never been to one of these and and I don't know if they charge for facilitator training, but how about checking with SENG regarding their parent groups? They have info on their site: http://www.sengifted.org/smpg_parent_groups.shtml

That might give you some tips for groups, though, at least! Good luck! How are you thinking of recruiting and finding people for this group?