Most cruises are during the European Summer months when there are many hours of daylight. It is possible in Winter but the weather can be nasty and it will be dark for many hours each day.
Since most cruise lines only cruise the Fjords for around three months during the Summer, You don't actually have a great amount of choice! It would be at it's lightest around now, and it's warmest during August I suppose. Do you want to avoid the school holidays? The earliest cruises, in May, tend to be the cheapest. Are you interested in spotting flora or fauna? Are there any local festivals, etc that you would like the cruise to coincide with? The (limited) choice is yours. Happy cruising .
Hello there - May and June are probably the best months for visiting the fjords - good light and lots of Spring blossom/flowers. July is also good but any later and you run the risk of cooler, wetter weather, although you can get this sort of weather any time in Norway. The fjords are, however, utterly beautiful and I'm sure you will enjoy them whenever you go. Regards - Janet.