What is Passover ?
Passover - also known as Pesach in Hebrew - is a major Jewish festival that principally celebrates the physical and political freedom attained by the Hebrew people following their exodus from Egypt where they had been slaves anywhere between 86 years and 116 years, depending on the scholarly conclusions one follows. The Hebrews had been in Egypt for 210 years with the final 116 years to 86 years before their Exodus from Egypt being as slaves. Technically-speaking, Passover or Pesach is a Jewish festival and not a Jewish holiday; a major or minor Jewish festival is distinguished from a Jewish holiday by its primarily being a joyous occasion in mood and theme. Furthermore, the three major festivals in Judaism are distinguished from the minor festivals in Judaism by their being commandments from G-d that are found in the Torah whereas the minor festivals were commanded by Jewish persons. Colloquially-speaking, however, Passover or Pesach, along with other major and minor Jewish festivals, have come to be referred to as Jewish holidays.
The highlight of the Passover festival is the festive meal which opens the festival. This festive meal is known as the Seder ("Order" in Hebrew). Jews who celebrate Passover for seven days hold the festive meal on the first evening of Passover while Jews who celebrate Passover for eight days hold two identical festive meals: the first on the first evening and the second on the second evening of Passover (the Jewish day begins and ends either from sunset to sunset or from nightfall to nightfall, depending on the authoritative rabbinical opinion one follows. Nightfall in Jewish law is declared when one sights three medium-sized stars in the sky; this can occur anywhere from about 20 minutes to 1 hour after sunset, depending on one's geographic latitude and location. If the sky is cloudy, nightfall in Jewish law is declared at 72 minutes after sea level sunset). Therefore, the Seder begins either immediately after sunset or immediately after nightfall, depending on which authoritative rabbinical opinion one follows concerning the start of the Jewish day.
Throughout the seven or eight days of Passover or Pesach, there are many Jewish laws and customs that are followed which are specific to the festival and the interpretation of these laws and the resulting manner in which they are practised depends on the opinion of the particular rabbi one follows which in turn depends on the Jewish cultural origins - whether Ashkenazi, Sephardi, or Mizrahi - and stream of Judaism practised by the rabbi - whether Orthodox, Conservative, Reform, Reconstructionist, or Humanistic, or any sub-division or offshoot within or between these streams, respectively. In addition, local customs and traditions of the Jewish community where one is located may also be added into the Passover rituals of that community, making certain Passover rituals unique to those communities.
Passover or Pesach is also commanded by G-d in the Hebrew Bible to be celebrated each year by the Hebrews, and later on in time, by the remnant of the Hebrews, the Jewish people. The commandment by G-d to commemorate Passover - the Torah of the Hebrew Bible refers to Passover as the Festival of Unleavened Bread (Chag Ha-Matzot in Hebrew) - annually is first mentioned in the biblical book of Shemot or Exodus, in Shemot 12:14,17 or Exodus 12:14,17 (Shemot or Exodus is the second book in the Hebrew Bible: the first five books of the Hebrew Bible comprise the Torah).