Norway became a unified independent Kingdom in 872 and entered a union with Sweden and Denmark in 1397. Sweden left the union in 1523, leaving Norway in union with Denmark until 1814 when Norway was transferred to a union with Sweden as a part of the aftermath of the Napoleonic wars. Norway regained full independence in 1905.
I am not sure which year Frozen is supposed to be set in, but I would assume either before or just after the Unification in 872.