structural:summarization
Date and duration

The Passover begins on the 15th day of the month of Nisan, which at present falls between March 26 and April 25 of the Gregorian calendar. The 15th day begins in the evening, after the 14th day, and the seder meal is eaten that evening. Passover is a spring festival, so the 15th day of Nisan typically begins on the night of a full moon after the northern vernal equinox. However, due to leap months falling after the vernal equinox, Passover sometimes starts on the second full moon after vernal equinox, as in 2016.

To ensure that Passover did not start before spring, the tradition in ancient Israel held that the lunar new year, the first day of Nisan, would not start until the barley was ripe, being the test for the onset of spring. If the barley was not ripe, or various other phenomena indicated that spring was not yet imminent, an intercalary month (Adar II) would be added. However, since at least the 4th century, the intercalation has been fixed mathematically according to the Metonic cycle.

In Israel, Passover is the seven-day holiday of the Feast of Unleavened Bread, with the first and last days celebrated as legal holidays and as holy days involving holiday meals, special prayer services, and abstention from work; the intervening days are known as Chol HaMoed ("Weekdays the Festival"). Jews outside of Israel celebrate the festival for eight days. Reform and Reconstructionist Jews usually celebrate the holiday over seven days.

Karaites use a different calendar; they rely on visual identification of ripe barley and the date of Passover cannot be determined before this. Some modern Karaites follow the Rabbinical calendar in modern Israel because of social pressure.

The Samaritans use a calendrical system that uses a different method from that current in Rabbinic practice; it sometimes is the same date on the solar calendar, sometimes two days later, and sometimes an entire month later. In 2024, Rabbinical Passover begins at sunset on 22 April. On the calendar used by the Samaritans, 22 April 2024 is also the day of the Passover sacrifice. Karaite and Samaritan Passovers are each one day long followed by the six-day Festival of Unleavened Bread, for a total of seven days.