Schedule for MySQL 4 Upgrade?

I’m starting to work myself into a dither over the upcoming MySQL updgrade. My sites are heavily dependent on third-party applications that use MySQL and while I’m trying to search around and learn what will happen to them (mainly pMachine, phpBB, vertiCal) I’d sure appreciate knowing on what day I will have to prepare to shut everything down if applications start choking. Since I’m sort of an ‘end user’ of these applications, some of the ‘prepare for upgrade’ mysql instructions I’ve been browsing seem more targetted at the application developer than me (although I could be wrong).