Spring is a rather complicated season with no clear timeframe, all depends on when snow coverage melts. It rarely thaws in March, practically never, so it is often regarded as a winter month.
Relatively spring-like weather is observed in Norway around Easter, which most often falls in April. At last snow melts, and every next day brings more sunshine and warmth. Nevertheless, some April days could appear quite cold as in winter, and sometimes, on the contrary, it is summer-like warm, but these are just exceptions. The month is also quite rainy. The best place to enjoy dry weather during April is the city of Oslo. All in all, the inland area is warmer than coastal regions as the land warms up faster than the sea.
Though this is not the peak of tourist season, yet May in terms of weather is quite nice time to visit Norway, especially its second half since the beginning is still a little cold. In addition to warm polar days, the month opens the season of wildflowers and bird watching. Melted snow and rainfalls provide great white water rafting, relatively dry remote trails are finally hiking-available.