When is the thirtieth day of the first month?

The 30th day of the first month coincides with the new calendar method, and the period is 19, which generally occurs every three years.

Sometimes two years in a row. In ancient times, there was an error between the calendar year and the revolution time of the earth. In order to make the 24 solar terms conform to the reality, the lunar calendar will increase or decrease the date appropriately. This is why sometimes the lunar calendar has 13 months in a year.