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there were originally 13 months of equal days.
The 13-month, 28-day alternative has been in use on this planet for more than 6000 years. In prehistoric India and China, and throughout South America it was the standard time-keeping system. The Essenes, Egyptians, Polynesians, Maya, Inca, Lakota, and Cherokee used a 13-month, 28-day calendar. The Celtic knowledge of the Druids is based on the Tree Calendar, also a 13-month, 28-day calendar. Today many cultures are still using their traditional 13-month calendar system.
https://blog.calendartruth.info/2017/03/22/history-of-the-13-month-28-day-calendar/