Originally it used to be celebrated on the 5th day of the 5th month (the old lunar calendar); at present, however, it is held on May 5 of the Gregorian calendar (the new calendar).
そもそもは5月5日(旧暦)に祝われたが、今日の日本ではグレゴリオ暦（新暦）の5月5日に行われる。