Why is 202 1 lunar October 65438+29 days?

In a year, 65438+ 10, March, May, July, August, 65438+ 10 and 65438+February are big months, with 3 1 day in each month, April, June, September and/kloc. 202 1 year is no exception. 202 1 month is a big month with 3 1 day, so 202 1 month has 3 1 day and cannot be 30 days. October in the lunar calendar has only 29 days. I just looked through the calendar and found that the lunar calendar 202 1+ 10 has only 29 days. But this lunar calendar is different every year, and it is not easy to calculate.