We are at about a 50% RSVP response and I put down today as the date. I can chase a couple of parents, but others I have no idea. I find family friends are the worst for RSVPing and then not coming if something else comes ups, which can be hard, as it means 2-3 are suddenly not coming.
The school RSVP's tend to be pretty good. I need to know numbers for the party entertainer, as she charges based on numbers and for making up party bags. I tend to over buy on food, so I put some out, then judge if more needs to go out.

I have had the odd invite handed over the night before, frustrating, but the other parent has been understanding when I have called.