The Roman Calendar
Because the Romans were an agrarian society, the phases of the moon and the seasons were the first yardsticks they used to build a calendar. The earliest known example supposedly dates from the founding of Rome (753 B.C.). That early calendar had ten months:
Martius (31 days), Aprilis (30 days), Maius (31 days), Junius (30 days), Quintilis (31 days), Sextilis (30 days), September (30 days), October (31 days), November (30 days), December (30 days)
Notice that December (Deci=10) is the tenth month, and there are only 304 days in this calendar. The other 61 days were not in a named month. Martius 1st (the first day of the year) was on the Vernal Equinox, which was the first day of planting for farmers.
In the 713 B.C, the calendar was reformed and two months were added at the end of the year.