A Runic calendar is a perpetual calendar based in the 19 year Metonic cycle of the Moon. The Greek astronomer Meton of Athens (Vth century BC) observed that a period of 19 years was equal to 235 synodic months and 6.940 days which is almost equal to 19 solar years except for a few hours. This cycle was used in the Babylonian calendar and Meton computed all the necessary parameters and the intercallary months to adjust the periods of the Sun and the Moon.
Runic calendars were written on parchment or carved onto staves of wood (as the one of the Estonian History Museum), horn or bone. It appears to be a medieval Swedish invention and the Nyköping staff, believed to date from the 13th century, is the oldest one which is preserved. The Runic calendar preserved in the Museum is dated in 1819 and its first line is made up of the first seven letters of the Runic alphabet (runes). 52 weeks of 7 days were laid out using 52 repetitions of this first seven runes and each rune corresponded to each weekday varied from year to year. On another line, many of the days were marked with one of the 19 symbols representing the 19 possible positions of a year in the Metonic cycle (called “Golden Numbers”).
This kind of calendars were used until mid-19th century.