Which civilization invented the 12-month calendar?

In the beginning, calendars were lunar-based because moon phases were easy to measure. In Sumerian culture, the year was divided into 12 months based on the lunar cycle.

One of the most famous ancient calendars developed by the Egyptians is the Egyptian or Sothic calendar. 365 days were divided into 12 months of 30 days, with five or six extra “epagomenal” days added at the end.

The Egyptians’ knowledge of astronomy contributed most to the development of this calendar.

A mesolithic arrangement of twelve pits and an arc found in Warren Field, Aberdeenshire, Scotland, dated to roughly 8,000 BC, has been described as a lunar calendar and was dubbed the "world's oldest known calendar" in 2013.