The Summer Solstice occurs on June 21. The maximum temperature in June hits 78.8°F, while the minimum temperature drops to 66.2°F. The average precipitation is 168.5mm, and rain is the most common type of precipitation. The temperatures aren't that different between winter and summer, but the amount of precipitation is where the seasons differ the most.
The Spring Equinox falls on either March 20 or 21. Due to the need for recent astronomical measurements, the exact date can't be determined until February of the previous year. In March, the average maximum temperature is 57.2°F, and the average minimum temperature is 42.8°F. Rainfall averages about 125 mm. Rain is the most likely precipitation to occur on the Spring Equinox.
The Autumnal Equinox falls on September 22 or 23. Again, the exact date cannot be determined until February of the year prior. In September, the average maximum temperature is 82.4°F, and the average minimum temperature is 69.8°F. The average rainfall is 215.1 mm and, again, rain is the most common type of precipitation.
If you are thinking about visiting Japan, I would recommend going in the winter. There are all sorts of special winter festivities going on all across the country, and most cities have something unique. There is the Sapporo Snow Festival in Hokkaido, which has all kinds of fancy ice art on display, along with winter sport events. Kyoto has Hanatoro, which is when the streets are illuminated with thousands of lanterns and are decorated with flower and light displays. Then there is the Sendai Pageant of Starlight, where rows of zelkova trees are decorated with hundreds of thousands of lights to create luminous corridors along the main streets of Sendai.
Be warned that sometimes heavy snows can hit, but this is mostly when you go more north or into more mountainous areas. Other than that, winter is the driest season in Japan, and the weather is nice and cold without being unbearably so. (Opinions may vary...I favor cold weather.)
Here's a goofy but catchy saying I came up with to advertise winter travel to Japan:
"Sapporo, Kyoto, Tokyo, wherever you go, witness spectacles in the snow!"
