Fuji TV, one of the major broadcasting companies in Japan, started the pattern of airing the dramas it predicted would be most popular on Monday nights. Mondays are the only night in which no baseball games are aired, meaning that throughout the year, there would be no delays in broadcasting (in Japan, due to the popularity of baseball, baseball game broadcasts take precedence over other programming, so shows are often pushed back due to late games).
While all of the stations often aired dramas on Monday nights, it was the popularity of the dramas Tokyo Love Story and The 101st Proposal that propelled the 9PM slot on Mondays. Fuji TV started branding its 9 pm Monday shows as "Getsuku", and actors and actresses would be interviewed as starring in their first "getsuku". Not long after this branding began, the other stations also started showing their most popular dramas on Monday nights.