Originally, '大掃除' was performed to clean the entire house at the end of the year to welcome the gods on January 1st. While fewer people today believe in gods, many still clean their homes to start the new year in a fresh and clean environment.
December 29th is avoided for '大掃除' because the number 9 in Japanese can be associated with '苦しい' (suffering), making it an unlucky day for activities related to gods or significant events.
Japan is unique in having a widespread tradition of thoroughly cleaning the house at the end of the year, which is not common in many other countries.
The best time for '大掃除' is around mid-December, with December 13th being considered the most auspicious day. December 29th is avoided, and it's recommended to finish by December 30th.
During '大掃除', areas that are usually neglected, such as high places, behind furniture like shelves and cabinets, and hard-to-reach spots, are thoroughly cleaned.