Runestaves. Museum Of History, Lund, Sweden
The Germanic peoples had their own names for the months which varied by region and dialect, which were later replaced with local adaptations of the Roman month names. Our records of Old English and Old High German month names date to the 8th and 9th centuries, respectively. Old Norse month names are attested from the 13th century. Like most pre-modern calendars, the reckoning used in early Germanic culture was likely lunisolar. The Runic calendar developed in Medieval Sweden is lunisolar, fixing the beginning of the year at the first full moon after winter solstice.
The month names do not coincide, thus it is not possible to postulate names of a Common Germanic stage, except possibly the name of a spring and a winter month, *austr- and *jehul-. The names of the seasons are also Common Germanic, *sumaraz, *harbistoz, *wentrus, and perhaps *wēr- ”spring”. The Common Germanic terms for “day”, “month” and “year” were*dagaz, *mēnō-þ- ”Moon” and *jǣrom. The latter two continue Proto-Indo-European *me(n)ses-, *iero- while *dagaz is a Germanic innovation from a root meaning “to be hot, to burn”.
Tacitus in his Germania (ch. 11) gives some indication of how the Germanic peoples of the 1st century reckoned the days. In contrast to Roman usage, they considered the day to begin at sunset, a system that in the Middle Ages came to be known as the “Florentine reckoning”. The same system is also recorded for the Gauls in Caesar’s Gallic Wars.“They assemble, except in the case of a sudden emergency, on certain fixed days, either at new or at full moon; for this they consider the most auspicious season for the transaction of business. Instead of reckoning by days as we do, they reckon by nights, and in this manner fix both their ordinary and their legal appointments. Night they regard as bringing on day.”
The concept of the week, on the other hand, was adopted from the Romans, from about the 1st century, the various Germanic languages having adopted the Greco-Roman system of naming of the days of the week after the classical planets, inserting loan translations for the names of the planets, substituting the names Germanic gods in a process known asinterpretatio germanica.
The months were probably lunar; the Old English ”mónaþ”, Old Norse ”mánaðr, and Old High German ”mánód”, as well as the modern English ”month”, modern Icelandic ”mánuður”, modern Norwegian ”måned”, modern Swedish ”månad”, modern Dutch ”maand”, and the German ”Monat”, are all derivatives of the word “moon”, with the -th suffix found in words such as “depth”, “width”, “breadth”, etc. This connection is also found in several other Indo-European languages.