The Modern Hebrew Calendar. The lunar months all begin near a new moon, and an extra month is inserted seven times every nineteen years before Nisan to keep that month in the spring. Note that Nisan is numbered as the first month (Ex. 12:2) even though the civil year begins with the planting season in the fall (Lev. 25:3-22) on 1 Tishri. The original seven holy days of the law of Moses are indicated with black dots. This modern calendar is based entirely on calculations; the Judean calendar used at the time of Christ was based on actual observations of the thin crescent of the new moon to determine the beginning of the month. For some purposes years are reckoned from Nisan, but the official civil year begins on 1 Tishri.
The modern Hebrew Calendar is a lunisolar ("luni"= moon, "solar"=sun) calendar, meaning that it starts its months at the new moon and uses years which are tied to the seasons of the sun. Each month has either 29 or 30 days, which are alternated as needed to keep in sync with the moon's 29.53-day period of phases. Similarly, either 12 or 13 months are included in the year to track the 365.24-day seasonal year. There is a more detailed description of lunisolar calendars in general in the Introduction to Calendars section, which describes the mathematics of how this is accomplished. The Hebrew Calendar has Several Holy Days, shown in the illustration.