Japan Travel Guide

Spring:  If you want to get the money shot during your 3 weeks in Japan, visit during the Cherry Blossom Festival, mid-March - mid-April. On the downside, it'll be crowded and accommodations will probably be pricey. You also don't have to visit right during the blossoming of the cherry trees. You can visit at the end of April and May and get perfect weather albeit not the Cherry Blossoms.

Fall: Fall is also a beautiful time to visit. The leaves changes color and the countryside is covered in reds, yellows, and oranges. The weather is also milder and thus, more enjoyable.

Summer: I went for 3 weeks to Japan in August, and if there is one thing that I am certain of in life, it's that you shouldn't go to Japan in August. It's hot. I mean it's really, really hot. Unbearably hot. Tokyo's concrete and August heat do not mix well. You also have to worry about it being typhoon season (June to October). You can get more information here on what happens when there's a major typhoon in Japan. 

Winter: Doing a 3-week itinerary in Japan can have its upsides and downsides. Japan gets snow in the winter in the Alps, making some destinations more difficult if not impossible to get to. But on the other hand, you can get some beautiful shots in of the snow. If you like skiing, this is a good time to go. If you like hiking, it's not. And since it's the low season, accommodations will be cheaper and tourist sights will be less crowded.