The calendar is based on the 19-year-long Metonic cycle, correlating the Sun and the Moon. Runic calendars were written on parchment or carved onto staves of wood, bone, or horn. The oldest one known, and the only one from the Middle Ages, is the Nyköping staff from Sweden, believed to date from the 13th century. Most of the several thousand which survive are wooden calendars dating from the 16th and the 17th centuries. During the 18th century, the Runic calendars had a renaissance, and around 1800, such calendars were made in the form of tobacco boxes in brass.
A typical Runic calendar consisted of several horizontal lines of symbols, one above the other.
The calendar does not prove knowledge of the length of the tropical year or of the occurrence of leap years. It is set at the beginning of each year by observing the first full moon after the winter solstice. The first full moon also marked the date of Disting, a pagan feast and a fair day.
On another, many of the days were marked with one of 19 symbols representing the 19 Golden numbers, the years of the Metonic cycle. In early calendars, each of the 19 years in the cycle was represented by a rune; the first 16 were the 16 runes of the Younger Futhark, plus special runes for the remaining three years: Arlaug (Golden Number 17), Tvimadur (Golden Number 18), and Belgthor (Golden Number 19). The new moon would fall on that day during that year of the cycle. For example, in the 18th year of the cycle, the new moons would fall on all the dates marked with Tvimadur, the symbol for year 18. Later calendars used Pentadic numerals for the values 1–19.
Because this system needed 19 runes to represent the 19 golden numbers which stood for the 19 years of the perpetual calendar's cycle, the Younger Futhark, a Runic alphabet, was insufficient, having only 16 characters. The solution devised was to add three special runes to represent the numbers above 16: Arlaug (Golden Number 17), Tvimadur (tvímaður, Golden Number 18), and Belgthor (Golden Number 19).
A primstav (translation: prime staff) is the ancient Norwegian calendar stick. These were engraved with images instead of runes. The images depicted the different nonmoving religious holidays. The oldest primstav still in existence is from 1457 and is exhibited at Norsk Folkemuseum.
- Becker, Alfred: A Magic Spell "powered by" a Lunisolar Calendar, Asterisk, A Quarterly Journal of Historical English Studies, 15 (2006)
- Becker, Alfred: Franks Casket. Zu den Bildern und Inschriften des Runenkästchens von Auzon (Regensburg 1973)