https://www.vice.com/en_us/article/v7be5b/a-mysterious-mayan-calendar-stumped-scientists-for-decades-a-new-study-has-cosmic-answers

Scientists have solved a longstanding mystery about a Maya calendar that includes 819 days, an unusual number that has puzzled experts for decades, reports a new study. 

A pair of anthropologists demonstrated that multiples of this number sync up with the rhythms of planets in the solar system that are visible to the naked eye, which underscores the astronomical sophistication of these ancient Pre-Columbian peoples.