Complex sentence: The Finnish Swedish Heritage Day is celebrated on the same day as Gustav Adolfsdagen in Sweden, the day that king Gustavus Adolphus of Sweden was killed at the Battle of Lützen in 1632.
Split into simpler sentences: The Finnish Swedish Heritage Day is celebrated on the same day as Gustav Adolfsdagen in Sweden, the day that king Gustavus Adolphus of Sweden died. He was killed at the Battle of Lützen in 1632.