Ive done it for the last 4 years I think. Not doing it this year though, due to illness.
It's a great day! I just open my house from midday (I do midday because it always falls on the same day as school open morning, so my colleagues can conveniently drop in on the way home for coffee and cake as a reward for having to go to school on a Saturday!)
I usually borrow the hot water urn from school too, which is a god send, so there's loads of hot water on the go all the time. I make several cakes and a couple of tray bake brownies or something like that. Always remember to do one gluten free and one vegan. I have friends who will only eat both of those anyway, but it gives everyone a go.
Make sure you have a big box of teabags and a big jar of coffee and obviously milk and sugar. Mugs - again, I clear out the cupboard at work on a Friday and return on Monday, but I also bought a load of plain white ones from the supermarket. Think they were only a couple of quid for 4.
Anything you want to know? Oh, raffle. When you invite people, ask them for donations for the raffle. I've normal got some stuff that I've saved up during the year or bought in th sales. You could always raffle off the last of the cakes anyway. Don't forget to buy raffle tickets though!
There's usually a couple of games in the pack you get and a table cloth, balloons and bunting. I like to buy green napkins, just to keep the colour scheme going and I have take away boxes too.