The Hebrew calendar is a “lunisolar” calendar, based on both the cycle of the moon around the Earth and the cycle of the Earth around the sun. Months are based on the moon (lunar months) and years are based on the sun (solar years).
Because a complete cycle of the moon is about 29-1/2 days, Hebrew months will alternate between 29 and 30 days. Twelve Hebrew months will have 353, 354, or 355 days.
The orbit of the Earth around the Sun is about 365-1/4 days. That means a year of 12 lunar months will come up anywhere from 10 to 12 or 13 days short of the solar year.
To solve this problem, in some years (known as “leap years”) a 13th month of 30 days is added. This happens about 7 times in 19 years. That way, the summer months are always in summer, and winter is always in winter.
Sound complicated? It isn’t, really. At one time, everything was based on observation – the first sliver of the new moon was physically sighted and a new month was declared. The position of the sun (equinox) determined when it was springtime, and the new was begun. But now (and since about 358 CE), mathematical calculations are used to predict the phases of the moon and position of the sun. This method is attributed to Rabbi Hillel II, and makes it possible for everyone around the world to be on the same page.
If you’ve been around many Messianic or Hebrew Roots groups, you probably know that the calendar is controversial. Not everyone accepts this calculated calendar, and some insist on various methods of starting the month or year. That means not everyone is on the same page. But this website isn’t about the calendar controversies, it is about encouraging people to Read The Gospels.