Watch the episode when Trunks leaves and Piccolo starts to tell the story of what's going to happen in the future!! Piccolo will say"on May 12, at 10:00 AM...." then Goku cuts him off and starts to explain it! Then before they leave Goku says"OK see you guys here on May 12 at 9:00 AM." That's the mistake!!! Watch it and you'll see
Oh course they would all be there early! they need to stretch and discuss their tactics and stuff! It would be pretty useless of them to arrive JUST as the androids do, they wouldn't be prepared!
Well of course they wanted to get there early. the Z fighters (except Vegeta, he doesn't care really) Want to beat the androids before any innocent people get hurt!