This event is called sennichi mairi (千日詣り), literally 1000 day visit, because visiting the shrine on this day is supposed to bring you 1000 days worth of good luck! Atago Shrine (愛宕神社) celebrates the day every year, so head there for lovely physalis alkekengi "Chinese lantern" plants (hozuki/ほおづき in Japanese) lined up along the paths.