2 + 2 = 5

Once upon a time, there was a country, and the people in this country had strong religious beliefs that influenced their lives in many different ways. They believed that God created the world from nothing, that God did it in six days and then rested on the following day. They accordingly divided time up into periods of six days for work and one day for rest. With this cycle, the day of rest was the day after the sixth day of work, but it was also the day before the first day of work. Thus, instead of calling the day of rest the seventh day of the week, they called it the zeroth day of the week, since that helped to remind them that it was God's day, the day that God did zero work and the day that they would do zero work in order to spend it praising God and dedicating themselves to doing God's will.