The most popular season to visit Norway is summer (mid-June through August), when the days are long and often sunny, and much of the country is on holiday. Daytime temperatures usually range from 15-20C (in the south) to 12-16C (in the north).
If you have some flexibility, we think the best times to visit Norway are in spring (May-mid June) and autumn (September-mid October), when the temperatures are a few degrees cooler, but the chance of rainfall is actually a bit lower, and there are fewer tourists.
December through March are the best months in Norway for snow sports and the Northern Lights (aurora borealis). February-March are usually the most reliable for snow conditions, and October-December are generally best for Northern Lights, because (being pre-ski season) the rates and availability are better, and the chances of seeing them are essentially the same.
NORWAY: BY AIR
From UK: direct flights to Oslo, Stavanger, Ålesund, Bergen and Trondheim from London and some regional airports, including Manchester and Edinburgh (for Oslo), and Aberdeen (for Bergen & Stavanger). Airlines include Norwegian, SAS, BA, Ryanair and Wideroe.
For Tromsø, you'll need to change planes - most likely in Oslo. Note that Oslo's main airport is Gardermoen, 45km north of the city, while the smaller Oslo Rygge and Oslo Torp are respectively 60km and 110km south of the city (on different sides of the Oslo fjord).
From the rest of Europe: flights from most capitals and some major cities to Oslo, and also to a few regional airports. For direct flights to Tromsø, options include Dusseldorf and Frankfurt (with Lufthansa or Eurowings) and Zurich (with Swiss).
NORWAY: BY TRAIN (& FERRY)
From UK: you can take the train from London all the way to Oslo, with 2 overnight stops (Brussels & Copenhagen) or possibly just 1 overnight (in Hamburg). For more info see Seat 61.
Or you can use a ferry for part of the journey, such as Copenhagen-Oslo (with DFDS) or Kiel-Oslo (with Color Line) or Hirtshals-Kristiansand (with Fjordline or Color Line).
There are currently no direct passenger ferries from UK to Norway.