Cool site. The calendar isn't working properly, though (you have to refresh to view events you add if they aren't on the current day). I also somehow ended up at the year 20070 at one point just by pressing the next link.
Regarding how to launch: I'd wait until you're ready first (this is obviously more like an Alpha launch, so just showing it to the YC community was a good first step). Once you are, you can do the normal digg and bloggers route, but even better is to figure out who are your target users and try to get in touch with them. Advertise on a site or in a publication they all read (try to negotiate and get this for free in exchange for something else). You can write a press release if it makes you feel good, but odds are no one's going to read it at this stage--it never hurts, though.
Otherwise, just try to get in touch with your users and ask them to spread the word. Sending a personal email to the early adopters would probably go a long way--ask them what they thought about it, what could you improve, then tell them you'd appreciate it if they could tell their friends. People like cool T-shirts too...
Definitely post on a Monday or early in the week if you can. Fridays are for bad news, no one cares on the weekend, or holidays for that matter. Bad idea to come out on January 1st if it's a mainstream release. Otherwise it doesn't matter too much.
Good luck.