day tour of Normandy from Cherbourg, France

Most of the organized tours recommended in Rick's France book and by other contributors on Forum posts tend to leave from Bayeux or Caen. www.normandy-sightseeing-tours.com is one that will pick you up anywhere "for a price." I believe that the same would be true for many private guides with their own vehicles; they might live closer to the beaches but agree to pick you up elsewhere. The Cherbourg Tourist Office web site does not look very promising. Renting your own car is of course an option, too.

Cherbourg is too far from the D-Day Beaches. As Laura suggests, you need to get to Caen or Bayeux for a D-Day day tour.
You're at the tip of the peninsula, you need to be southeast.