Calvados is the famed apple brandy from Normandy. The equivalent in Brittany is Lambig. If you can't find either, you could use cognac, Armagnac, Applejack, or another brandy. Of course, you'd have to change the name of it...especially if you were serving it to someone from Normandy or Brittany!

Versions do vary and I've seen some recipes without the brandy and others that use creme de peche (peach) or creme de mure (blackberry) in place of the creme de cassis.