A brief comparison of the Gregorian and the Hebrew calendars will help us understand this subject.
The solar-based calendar commonly used today is of Roman origin. It contains about 365 1/4 days. And though the word month means "moon," the months are not governed by its phases. They are of arbitrary beginning and length.
On the other hand, the Hebrew calendar is lunisolar, that is, based on both the sun and the moon. The length of the year differs significantly, and the months are directly related to the phases of the moon.
In the Hebrew calendar, twelve lunar months result in a year that has about 354 days (about eleven days shorter than a solar year). Such common years, as they are called, are regularly balanced by leap years, which contain thirteen months each. Leap years are about 384 days in length (about nineteen days longer than a solar year). Notice how these lunar months are related to the solar year.
Every nineteen solar years, the moon revolves around the earth 235 times. In other words, 235 lunar months equals about nineteen solar years. This remarkable astronomical relationship makes it possible to combine twelve common Hebrew years (of twelve months each) and seven leap years (of thirteen months each) together every nineteen years. This means that the solar (Roman) calendar and the lunisolar (Hebrew) calendar very nearly coincide as the sun, moon and earth return to their approximate position of alignment with each other every nineteen years.
Nineteen-year patterns can be seen in biblical history. For example, ancient Israel spent 38 (19 x 2) extra years wandering in the wilderness (Deuteronomy 2:14; Numbers 14:33-34). Early church history divides itself into roughly three nineteen-year periods from Jesus' death to Paul's warning of apostasy (Galatians 1:6) to the church's flight from Jerusalem to the apostle John's exile to Patmos.