I'm in the preliminary stages of planning a Japan trip.

When is the "best" time to go to Japan? We can postpone...but would prefer this June.

Also, what is the weather like between Tokyo and Kyoto during June-August?

Would 10 days be enough for a first trip?

Thanks for the help!

If you can choose from June through August, the earlier the better as it becomes very, very hot and very, very humid in summer..Kyoto more humid than Tokyo but both hot. It may be rainy in June since it "rainy season" but in my memory of 10 years living in Japan, it rains more in November/December than in "rainy seaon". Personally, I'd choose rain over the heat and humidity any time.

The best time to go to Japan depends on you, your schedule, your expectations in terms of weather and holidays, and mother nature.

10 days is enough if that's what you have. We all always wish we had more time for every trip.

I have read so much about the steamy heat in summer that I am pulling my son out of school for an extra week before his spring break for our first mid to late March trip.
In late fall you get the leaves changing and in late March/early to mid April you get cherry blossoms (all the timing depending on weather and rainfall through the year and where you are in Japan).
We are going for 12 full days (aside from travel days) and those days are pretty full already, although we've set aside time for things that will just come up so we can follow our noses a bit. From what I've gathered doing the research for the trip, it depends on how far you want to go, and how much time you want to spend traveling around. Some people like to move every night or two, and some prefer to stay in just a few places and do day trips. How much you can do and where you can go depend on your travel style and what you are comfortable with, and personally I take into account how I respond to jet lag too.
If you are going to stay in Tokyo and Kyoto, look into going into Narita and out of Kansai airports, so that you don't have to go back to Narita from Kyoto to depart, like moi.

Early June was a fine time last year to be visiting central Japan. I was in Hiroshima, Takayama, Matsumoto, and Tokyo. Warm and not humid. Got some rain, but it was short and light rain, but mostly it was nice weather.

Of the summer months, you definitely want to go early (June) rather than later.

It will start heating up in mid June, but odds are you'll have decent weather...some rain or gloom possible, but sunshine too. July and August are just brutal - specially in Tokyo with all the concrete.

Like Kim said, 10 days is plenty if thats all you have!