Even Apple doesn't know for sure yet. They probably have a target date in mind, but it's not definite in any way. As they get closer to a Golden Master/RTM seed maybe they will announce a date. Anything at this point is just a guess.
I'm going to go out on a limb and predict Sept. 25 as the release date. This is based on the release date of Leopard on the last Friday of Oct. a couple years ago.
Tuesday September 09, 2008 - iPod event - 2nd Tuesday of month.
Wednesday September 05, 2007 - 1st Wednesday of month
Tuesday September 12, 2006 - 2nd Tuesday of month.
Not a bad guess for an iPod event, but Snow Leopard will v likely as Tallest Skill points out - have it's own event.