Complex sentence: In Sweden it is called "brittsommar", which is derived from Birgitta and Britta, who have their name day in the Swedish calendar on October 7, when Britt Mass, an official fall open-air market, was held.
Split into simpler sentences: In Sweden it is called "brittsommar", which is derived from Birgitta and Britta, who have their "name day" in the Swedish calendar on October 7. That is when Britt Mass, an official fall open-air market, was held.