From the MozillaWiki:

Proposed Freezes ( All dates use time of 23:59 Pacific )

• Slushy String freeze date: 2009-04-24 (Friday)
• Slushy Code Freeze date: 2009-04-26 (Sunday)
• Firm String / Code freeze date: 2009-04-28 (Tuesday)
• l10n-mozilla-1.9.1 freeze date: 2009-04-30 (Thursday)
• Target Build Ship date: 2009-05-05 (Tuesday)

Not sure what’s after this if is going to be a Beta 4 or may be if we are lucky Release Candidate 1!