A day and a month for interiors

The month of April is designated as "Interior Month" in Japan, and the fourth Saturday of every month as "Interior Day". In addition, April 10th annually is A Day to think about Interiors インテリアを考える日 (interia wo kangeru hi).

The reason is because the kanji for the tenth is associated with "dwelling" as it has the same sound. In addition, the date is also strategic as April marks a season of new beginnings in Japan - new school years, new jobs, new homes - making it a period when people naturally consider their dwellings. 

In English "interior" can also mean inside or inner, but in Japanese it specifically refers to the furnishings and decorative elements that shape a room. Including things like curtains, carpets, lighting, furniture etc. What we call "interior design" in English.