Could it be because, according to https://en.wikipedia.org/wiki/1320 "In France, a large group of common people band together in Normandy on Easter Sunday to begin a crusade, after a teenage shepherd says he was visited by the Holy Spirit" (my emphasis).
Regarding the year dating, more likely to be a reference to just the general medieval theme. Only event in the year (in Sweden) I could find that is notable is the execution of Magnus Birgersson (Heir apparent). Not sure why'd they reference that though, so I'm guessing it's just a number that is "close" to 133 + in the medieval times.