The number of tours to Norway in May indicates the beginning of the season. It's already warm enough, with almost no rain. Most ski resorts are closing, except for the northernmost ones. Budø, Tromsø offer interesting tours. You can come to cold Alta to see real rock paintings. Visit the town of Svolver and the Lofotr Viking Museum in the Lofoten Islands.
So, a late spring vacation in Norway is the start of the season for sightseeing, sailing, and, for extreme sports enthusiasts, even swimming on the beaches of Oslo and other more or less southern cities. Many cruises leave from Bergen along the coast, and to Svalbard, where it also gets a little warmer.