Oosouji (大掃除) oo means big and souji means cleaning. Oosouji is similar to a western style ‘spring clean’ performed around the 28th of December as a new year tradition.
It is a time in which you clean things not normally done during the year for general weekly or daily cleaning such as dusting under or behind furnishings including lounges, stoves and beds, washing windows, fans and light fittings or sorting through old linens and clothes. Normally the whole household will contribute to the cleaning together.
The main reasons for undertaking an oosouji are that it welcomes in the new year with a fresh clean home, and also because it prepares the house to welcome in toshigami (the god of shinto). The idea is that having a clean environment is starting the new year fresh without clutter and welcoming in the gods for another year with respect.
Oosouji is not just a household tradition but is also a tradition of many schools, where parents, students and staff help to clean and organise the school for the new year. Oosouji is also performed at the end of every school semester, so the school is ready for the new semester when the students and staff return. Due to Oosouji being performed every semester the clenning is not as intensive, however throughout the year more time would have been spent cleaning school than home.