The calendar is based on the 19 year-long Metonic cycle, correlating the Sun and the Moon, but the calendar does not prove knowledge of the length of the tropical year or of the occurrence of leap years. The two sliding halves are aligned and set at the beginning of each year by observing the first full moon after the first new moon after the winter solstice. The first full moon also marked the date of "Disting", a pagan feast and a fair day also known as Thing of all Swedes.