Why does the lunar calendar change every year?

The "new calendar" you mentioned is the Gregorian calendar, also called "solar calendar" and "western calendar", which is a common calendar all over the world.

Gregorian calendar is a calendar in which the revolution of the earth marks the year and the rotation of the earth marks the day. The period of revolution of the Earth (one year) is 365.25 days, one day shorter every four years. In order to meet the needs of adjustment, one day is added in February every four years. Four years is one day away, very accurate.

The lunar calendar is actually a kind of "lunar calendar", in which the revolution of the earth marks the year, the revolution of the moon marks the month, and the rotation of the earth marks the day. Because the period of revolution of the moon is about 29.5 days, the number of days per year is not 365.25 days, and there will be fewer days. In order to adjust, seven "leap months" are added every 19 years, which is called "seven leap months in nineteen years". A year with a leap month is called a leap year, and a leap year has 13 lunar months. In this way, the lunar calendar and the Gregorian calendar are in tune. So every month of the lunar calendar always changes.

Lunar calendar and Gregorian calendar overlap once every 19 years.