The 15th day of the lunar calendar falls on the Kishuku-nichi day (when oni or ogre do not walk around and it's a very lucky day for everything except for weddings) of the Nijuhasshuku (Twenty-eight mansions, a date counted in Chinese astronomy), and the day is said to be of Good Fortune for everything.
旧暦の15日はかつては二十八宿の鬼宿日（鬼が出歩かない日）に当たり、何事をするにも吉であるとされた。