St Olav Festival is a celebration of Norway’s patron saint and his crucial role in bringing Christianity to Norway. St Olav, who was king of Norway from 1015 to 1028, famously died at the Battle of Stiklestad on 29 July 1030. Today his remains are buried in Nidarosdomen, Trondheim’s cathedral, and Norway’s most important church. The festival, which lasts over a week in the summer, has a religious dimension, with pilgrim wanderings and daily services in the cathedral, but it also offers concerts, theatrical performances, lectures and exhibitions, as well as a historical medieval market.

Source: https://theculturetrip.com/europe/norway/articles/10-festivals-you-will-only-find-in-norway/