I'm 20th August, and I just sent emails to those invited to the day telling them the date and venue with details of room booking and stuff. It's not perfect, but there you go. I can't go back in time and send save the dates when I should have done (ie once we'd booked the venue!) Anyone I didn't have an email addy for I'm just sending a simple letter by snail mail.
I know loads of people will think this is awful, but for those of us who are organisation impaired, sometimes good enough just has to be good enough!
Am on the invite trail though. Going to get them out asap. Good luck!