Passover is an eight day Jewish holiday, of Biblical origin (one of the 4 holidays during the year that the bible says are to be celebrated for all generations). It celebrates the liberation of the children of Israel from slavery in Egypt over 3000 years ago, under the leadership of Moses.
Passover celebrates the exodus from Egypt. Had the Jews not left Egypt, their religion (and the Jewish people) would not have survived and the religions that grow out of Judaism, such as Christianity, would never have happened.
Passover is also a time of convergence between the Jewish and Christrian religions. The Last Supper was a Passover Seder and the early Christrians continued to celebrate Passover for hundreds of years. Today there is a growing movement among Christians to return to a celebration of Passover.
The Seder is a ritual banquet that reenacts the Exodus. Its major feature is the reading of the Haggadah, which relates, in detail, the events of the Exodus of the Jewish people from ancient Egypt, complete with symbolic reenactments using wine (four cups), matzoh (unlevened bread), and bitter herbs.
The Haggadah is a small book that is used at the Passover Seder. It contains the order of the Passover Seder and is used by the Seder leader and participants to conduct the rituals of the meal. I have written my own Haggadah that is different for "traditional" Haggadahs. My Haggadah attempts to reconstruct the Passover Seder it was celebrated in biblical times.
During Passover it is forbidden to eat anything that is leavened or that could become leavened. Such foods are called chametz. Specifically, the bible defines chametz as anything that:
The short answer is yes. Three of the four Gospels make it very clear that the last supper was Passover (see Matthew 26 verses 17 to 21, Mark 14 verses 12 to 18, and Luke 22 verses 7 to 15). Only in the Gospel of John it is not clear if the Last Supper was Passover or the night before. So, if the majority of the Gospels are so clear about the Last Supper being Passover, why is there confusion?
The long answer is complicated. We know from history that the early Christians wanted to distance themselves from the Jews. Around 190 AD Pope Victor issued an order that Easter must be celebrated on Sunday instead of the 14th day of the moon (Passover).
Easter was further distanced from Passover when, in 325 AD, the First Council of Nicaea decreed:
We also send you the good news of the settlement concerning the holy pasch, namely that in answer to your prayers this question also has been resolved. All the brethren in the East who have hitherto followed the Jewish practice will henceforth observe the custom of the Romans and of yourselves and of all of us who from ancient times have kept Easter together with you.
Another part of the answer lies in conflicts between the Gospels and the later books of the New Testament. Here the answer is not as clear. Some people believe that the First Council of Nicaea selected the books that were to be part of the New Testament and rejected (some say destroyed) books that conflicted with their views. I have not found historical evidence that supports that view and the internal conflicts between books in the New Testament would also seem to argue against it.
Despite these conflicts, the answer is clear. There would not have been a reason for Pope Victor to fix the date of Easter to Sunday or the First Council of Nicaea to make its decree unless the Last Supper and Passover were the same event.
The early church was faced with the following conflict in dates: Jesus rose on a Sunday, but Passover can fall on various days of the week. So the early church saw two options:
Although the issue was hotly debated and variously practiced during the first centuries of the church, the Council of Nicea in 325 A.D. eventually adopted the current system of celebrating Easter on the first Sunday that follows the paschal full moon, which is the full moon that falls on or after the spring equinox. Note that this specifies date for Easter in two different ways:
The Council of Nicea believed that these dates were identical, and they were until, in the 1582 when Pope Gregory XIII, adapted the Julian calendar. Under the Julian calandar spring Equinox was defined as first full moon which occurs on or after March 21. Mostly this agrees with the date of Passover, but not always (which is why a good Standard never defines the same thing in two different ways).