The Passover begins on the 15th day of the month of Nisan, which typically falls in March or April of the Gregorian calendar. Passover is a spring festival, so the 15th day of Nisan begins on the night of a full moon after the northern vernal equinox. To ensure that Passover did not start before spring, the tradition in ancient Israel held that 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 date has been fixed mathematically.
In the Israel, Passover is the seven-day holiday of the Feast of Unleavened Bread, with the first and last days observed as legal holidays and as holy days involving abstention from work, special prayer services, and holiday meals; the intervening days are known as Chol HaMoed ("Weekdays [of] the Festival"). Diaspora Jews historically observed the festival for eight days, and most still do. Reform and Reconstructionist Jews and Israeli Jews, wherever they are, usually observe the holiday over seven days. The reason for this extra day is due to enactment of the ancient Jewish sages. It is thought by many scholars that Jews outside of Israel could not be certain if their local calendars fully conformed to practice of the Temple at Jerusalem, so they added an extra day. But as this practice only attaches to certain (major) sacred days, others posit the extra day may have been added to accommodate people who had to travel long distances to participate in communal worship and ritual practices; or the practice may have evolved as a compromise between conflicting interpretations of Jewish Law regarding the calendar; or it may have evolved as a safety measure in areas where Jews were commonly in danger, so that their enemies would not be certain on which day to attack.
Passover is divided into two parts:
The first two days and last two days (the latter commemorating the splitting of the Red Sea) are full-fledged holidays. Holiday candles are lit at night, and kiddush and sumptuous holiday meals are enjoyed on both nights and days. Jews don’t go to work, drive, write or switch on or off electric devices. They are permitted to cook and to carry outdoors.
The middle four days are called chol hamoed, semi-festive “intermediate days,” when most forms of work are permitted.
Source: wikipedia.org | chabad.org
In 2018 Passover in USA falls on March 31.