A quick browsing of the Y! message board someone says it's August 14th, 3am.
Not to hijack this thread, but does anyone want to join a dynasty keeper fantasy football league? Here's the league http://games.espn.go.com/ffl/leagueoffice?leagueId=304392 to check out the settings.
The keeper rules are as follows: You get a maximum of 8 keepers per off-season. One keeper can be ANY age, the other 7 must be 27 or younger on or before September 1 of that year. Hence the "dynasty" title.