First, I agree with the other ladies that no matter when you plan it, there will be those that can't come.
I think that Labor Day gives people the best opportunity to come, especially if they are out-of-town.
The folks that are still travelling will either decide to stay in town this one time or stop by another time. I would hint around to your friends as soon as possible to get an idea of their plans. Be sure to tell them it's a surprise (and don't tell them if you think they can't keep it under wraps... better to miss them attending than have them ruin the surprise).
As for the Saturday/Sunday question, I would think Saturday would be better, especially for those with a child (or two) and for those that work. Sunday would work if you did an afternoon affair.
I hope you have someone that will be able to help you with the setup and planning and all...
Have fun! And congratulations on your new addition! Hope it all goes smoothly with the c-section.