Introduction to the Yi Eighteen-month Calendar Monument:The Yi Eighteen-month Calendar is one of the oldest calendars in the world, with a history of tens of thousands of years. This calendar divides the year into 18 months, each month has 20 days, totaling 360 days, plus 5 sacrificial days, making the whole year 365 days. The 18-month calendar of the Yi people is strikingly similar to the 18-month Mayan calendar, which is of great significance to cultural and anthropological research.