The first day of Sanja Matsuri can fall in Sensō-ji, Tokyo
The first day of Sanja Matsuri can fall in Sensō-ji, Tokyo is held on May 19. Celebrated on the third weekend of May. This event in the second decade of the month May is annual. 
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The three men became known as “Sanja-sama” (Takenari and Hamanari Hinokuma – and their friend, Matsuchi Hajino who founded Senso-ji Temple), and the festival became known as Sanja Matsuri in their honor. It is also believed that a festival was held from 1312 in honor of the Sanja-sama, but this only took place every other year in March until 1649.
Every year huge numbers of people attend the Sanja Matsuri (三社祭), one of the three great Shinto festivals in Tokyo held annually for 3 days on the third weekend of May in Asakusa Shrine.
Similar holidays and events, festivals and interesting facts
Buddha's Birthday on May 15 (Buddhist festival - 灌仏会 - Matsuri, which is held in Japan. It is also referred to as Hana Matsuri, "Flower Festival");
Aoi Festival in Japan on May 15
Japan Day in Düsseldorf on May 25 (date for 2024. Celebrated on the last Saturday of May);
Navy Anniversary Day in Japan on May 27 (海軍記念日, Kaigun Kinen'bi. It was in commemoration of the Battle of Tsushima - May 27–29, 1905);
Dragon Quest Day on May 27
Kimono Day in Japan on May 29 (Gofuku No Hi);
Koromogae in Japan on June 1 (「衣替え」)