The big events seem to be on Saturday, April 16th and Tuesday, April 19th.
The Concord parade is on Monday however along with a whole host of activities in Lexington beginning at 6:30 AM. Before I get myself in more trouble, here is the link to the National Park website. My advice is the call the National Park. Look under contact info and hit zero to speak to a ranger and verify your schedule.
The good news is that the is an entire week's worth of cool stuff leading up to it. Check the sked at the the NPS and the Lexington link posted by a previous poster. It is very hard to get a bead on when all of this happens as each town has its own schedule of events.
Apologies for my earlier error stating the battle at the Old North Bridge happens on Monday.