Beltane (May 1st)
May 1st falls in the middle of a period known as “Golden Week” in Japan, which consists of four days of public holidays – Shōwa Day (Shōwa no Hi, April 29th), Constitution Memorial Day (Kenpō Kinenbi, May 3rd), Greenery Day (Midori no Hi, May 4th) and Children’s Day (Kodomo no Hi, May 5th). May 1st is not a public holiday, but some organisations give workers this day off anyway. Neopagans who don’t get this day off could consider holding their celebrations on the public holidays closest to Beltane (Shōwa Day or Constitution Memorial Day). Alternatively, Greenery Day is a secular day dedicated to Japan’s nature, so this too seems appropriate. Finally, Children’s Day is a Shinto festival that actually has some points in common with Beltane, which you can read about here. For these reasons, any day in Golden Week seems appropriate for celebrating Beltane!
Summer Solstice (around June 20th)
Not really observed in Japan except at specific places; you can read more about the Summer Solstice in Japan here. There are also no public holidays in June. However, between the Summer Solstice and Lammas there is Tanabata, which some Pagans might enjoy celebrating…