David came to his conclusion by looking at the Moscone Center schedule.
However, as I looked at the schedule, I noticed another generic Corporate meeting scheduled for Moscone West -- for June 6-June12. Thats the second week of June, the correct number of days, and ends on a Friday (WWDC nearly always ends on a Friday).
Yes, June 6 is a Saturday, but last year a similar generic listing on the Moscone Web site had the event starting on Sunday June 8 when the official start date was actually June 9. That means WWDCs official start date this year probably will be June 7, a Sunday unusual but not unprecedented. Back in 2005 WWDCs official first day was a Sunday.
Since the keynote is typically delivered on a Monday its possible that June 8th would the day Snow Leopard is announced. This would fall right in line with CEO Steve Jobs estimation that Snow Leopard would ship in about a year.