It’s Sunday, and it comes after Saturday because of arithmetic. Take the seven moving lights the ancients knew, in the order they believed them to sit, farthest to nearest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Give each hour of the day to the next one in line, starting with Saturn, and keep going round. Twenty-four hours is three full turns of seven with three hours left over, so the next day’s first hour lands three places down the list, on the Sun. Do it again and you reach the Moon, then Mars, Mercury, Jupiter, Venus, and Saturn again. That is the whole week in order, and Cassius Dio set it out this way in the early third century. Write the seven names round a circle and join every third one, and you draw a seven-pointed star. English swapped a few of the gods for northern ones along the way, Tiw for Mars and Woden for Mercury, but kept the order. Hardly anyone now believes a planet rules the hour, and we all still walk that star, one point a day.