On July 22 2011 Anders Behring Breivik murdered 69 adults and children on the island of Utoya.
He was also behind a car bomb at the government quarter in Oslo that killing a further eight people.
Prime Minister Erna Solberg has described the event as "a very sad" day that shook the small nation.
She told Norwegian news agency NTB "it is always very onerous".
On Friday she will lead government tributes at wreath-laying ceremonies and at a service in Oslo Cathedral.
Members of Norway's royal family, victims' relatives and representatives from a youth group that organised the summer camp on will also be attending the church service.