Late April/early May
At that time of year, I think you will have to rent a car.
From Oslo you can take the train as far as Åndalsnes or a flight to Ålesund, from Bergen Hurtigruten, bus or flight to Ålesund.Edited: 1:12 am, October 06, 2018
Early May is a fine time around fjords. Green below and white above
But the two iconic mt passes (Trollstigen and Geiranger/Dalsnibba) are still closed because of snow, note the date on this video
Getting to Geiranger at that time of year is a bit cumbersome