Shichi-go-san (七五三: 7-5-3) is a Japanese festival celebrated for children aged 7, 5 and 3, thus the name. The official date is November 15th, but as it’s not a national holiday most families will hold their festivities on the nearest weekend instead.
Parents of a 3-year-old or 5-year-old son and / or of a 3-year-old or 7-year-old daughter will take them to a local Shinto shrine. There, they pray to Ujigami (氏神), the Shinto guardian god of good health, wishing for the well-being of their children.
Usually a purification rite (harai, 祓い) and the reciting of a Shinto prayer (norito, 祝詞) are performed on that day. https://zoomingjapan.com/culture/shichi-go-san/