I had asked same question on HN before, and PG advised me that - the best launch day is when you are ready to launch.

Also, I will suggest don't focus on a single launch day. Think every day is a launch day. First launch on forums like HN, then get some feedback, then launch on few mid-size blogs, see if they cover it, see if you can handle traffic, then again launch on bigger blogs, and then keep launching to every single person you meet on each and every day. Once you are ready, every single day is a launch day.