On the traditional Japanese lunar calendar, the first day of the boar of the tenth lunar month was celebrated as the Day of Boar, when a special mochi cake called inoko-mochi 亥の子餅 (lit. baby boar cake) was eaten, specifically at the hour of the boar between 9 and 11pm, to bring good health and many descendants. On the tea ceremony calendar it marks the day when a special winter floor brazier is used for the first time, signalling the start of the cold season. This custom transferred into the domestic sphere as the day when the kotatsu (under table heater) was brought out for winter.
This tradition stems from the Gencho Ceremony 玄猪, originating in the Heian Era (794-1185), when, on this first day of the boar of the ten lunar month, the Emperor would make a ceremonial rice cake at the Go-ō Shrine 護王神社, located directly opposite the west gateway to the Imperial Palace. It was where the protective deity of the Emperor was enshrined.
This year, the first day of the boar is November 15th, but specialist sweet shops around Kyoto will be selling inoko-mochi throughout November.
Inoko Festival by Find Your Japan