if i know the person well or see them often, i'll usually rsvp straight away but if don't know them so well, i may leave it till closer to the rsvp date because i'm usually in a rush & think 'i'll do that later' but always by the rsvp date. did you give them a cut off date for rsvps?

i don't think 10am saturday is necessarily a bad time (although i don't have school age children so dunno what their activities might be like?) but if DS had (for eg) his swimming, i'd probably skip it that week so he could go to a friend's party or would let you know that we might be a little late. it also leaves the afternoon free for people so if there was something they needed to get done, they can still do it!

and you're not a total social retard!