Jungle Camp Mini World Change is a popular in-game event that takes place almost daily. You can then meet two Nemesis-type bosses - Arthom the Hunter (Hunter) and Oodok the Witchmaster as a Dworc boss. Their appearance is also one of the relatively frequent ones, but I wonder if these bosses can appear more often than once on a given day?
EDIT: To clarify, by writing "day" here I mean the period from SS, to SS the next day.