According to legend, a statue is said to have been discovered on the 18 March 628 on the riverbed of the nearby Sumida River by two fishermen. The village head, upon seeing the statue, recognized it as the Goddess of Mercy (Guanyin) and built the temple in her honor. The Honzon Jigen-e commemorates this date.
Incidentally, the Sanja Matsuri of Asakusa Shrine, which celebrates the three deified founders of Senso-ji, used to be held on the same day until the government decreed that Buddhist and Shinto practices must be separated in 1872. Today, the Sanja Matsuri is held in May (and is one of Tokyo’s three major festivals), and so on the night before (17 March) the spirits of the three founders are put into three mikoshi (portable shrines) and carried to the main hall of Senso-ji so that they can be reunited with the Goddess of Mercy before being carried back the following day.
The ritual starts at 6pm with the spirits being placed in the mikoshi and the shrines and carried to the main hall of Senso-ji (do-age) at 7pm. The mikoshi are carried back at 9:15am the following day and the procession is an extended one which goes around the Asakusa area if the 18th falls on a weekend. The kinryu no mai (Golden Dragon Dance), which celebrates the re-construction of the main hall of the temple, is also held on the 18th.