The best time regarding when to go is early summer, particularly June. Around June is when to go to if you want to experience outdoor events (like Midsummer’s Eve) and activities, warm weather, and long hours of daylight called the Midnight Sun. Also, May and July are great alternatives. However, May can be a bit more rainy than June, and while July is warmer, it also has larger tourist crowds. July and August can also be slightly more expensive that the earlier summer months, so June is still your best bet, as far as when to visit.
If you’re a budget traveler trying to decide when to go, I’d highly recommend late August and September. The flights to Sweden will be cheaper than during the high season, along with local hotel prices. And during that time, the weather is still mild enough for outdoor attractions, although some sights begin to close during September.
As a winter traveler it’s more about when you can avoid the crowds in the winter sports areas. The busiest times at ski resorts are late February, March, and December. It’s really idyllic during Christmas, though, plus you can visit the romantic Christmas markets in the bigger cities.
And it’s also the best time to see the Northern Lights in northern Sweden.