There are rabbis and scholars who are convinced that half of the biblical holiday of Passover was celebrated during the biblical holiday of Chag Hamatzot.
I heard a rabbi and professor mistakenly tell his audience that the biblical holiday of “Passover” of the fourteenth day of the first month of the year, later called Nissan, and the “Festival of Unleavened Bread” that began on the following day, the fifteenth, created a unique situation. The Torah mandated that the Israelites sacrifice and eat the Paschal Lamb on 14 Nisan on a day called Passover and they can eat it all night but no later than the next morning. Passover is a totally different holiday from the “Festival of Unleavened Bread” (Chag Hamatzot) which started on the fifteenth of Nisan the day following Passover in which the Israelites were to eat no unleavened bread.
The rabbi pointed out that “everybody knows” that in Judaism, the day begins in the evening with the setting of the sun. Thus, since the Israelites were allowed to eat the Pascal sacrifice during the night following the sacrifice on the fourteenth, they were observing the Passover holiday during the time that the holiday of the “Festival of the Leavened Bread” began at sundown. Thus, the Torah created a strange situation that two holidays were observed simultaneously. He thought he was clever and added that this was the origin of the statement “Why is this night different than all other nights?”
The problem disappears once it is realized that according to the Torah the day begins in the morning. It was only in post-biblical times that Jews developed the idea to start the day in the evening. In the Bible, the holiday of Passover began on the morning of the fourteenth and lasted during the night when the Israelites could still eat the Pascal sacrifice. Passover ended the morning of the fifteenth when Chag Hamatzot began. The two holidays did not occur at the same time.
(As I wrote in the past, the biblical holiday of Passover which occurred in 14 Nisan was a holiday whose sole observance was the sacrifice and eating of the Pascal sacrifice. When the temple was destroyed in 70 CE, sacrifices ceased. Since the Pascal sacrifice could no longer be brought, the holiday of Passover on 14 Nisan ceased as well. The name Passover was then given to the biblical holiday Chag Hamatzot. However, the Siddur did not accept the change of names, and the holiday is continued to be called Chag Hamatzot in prayers.)
Rashbam explains when the day begins
Rabbi Samuel ben Meir, known as Rashbam (c. 1085–c. 1158), a grandson of Rashi, was the author of a superbly rational commentary on the Bible and Talmud. Rashbam wrote in his commentary on Genesis 1:5 that the biblical day began at daybreak. The Torah states in Genesis 1 that God performed certain acts of creation on the first day; then there was evening and then morning when the first day ended, and God began new activities in the morning of the second day.
When did Jews change the idea when a day begins?
Apparently, during the exile of Jews to Babylon in 586 BCE, they discovered that the Babylonians began their day at nightfall and copied their practice.
It is known that the Judeans living in Babylon altered their calendar, clearly due to the influence of the community in which they lived. The Judeans also changed the biblical rule mentioned in Exodus 12:2 – that the month that was later called Nisan, the month containing the holiday of Passover, was the first month of the year. They named the seventh month the beginning of the year and later called the first day of the seventh month Rosh Hashanah, “New Year,” a name that this day was not given in the Bible. This was done despite the more conservative view of some talmudic rabbis who insisted that the world was created in Nisan. The Judeans also altered the names of the months, and the current “Hebrew” names of the months are actually Babylonian, even including the name of a Babylonian deity as the name of one month, Tammuz.
When did the day begin in the Temple before and after 586 BCE?
We know for certain that the day began in the Temple at daybreak and it is assumed that the priests in the Temple retained the ancient practice for as long as the Temple existed, until 70 CE.
Additionally, scholars believe that the Sadducees, who lived during the later centuries of the Second Temple period, continued the ancient custom of starting the day in the morning, and some Karaites, a sect that started in the ninth century and that followed many Sadducean traditions, did the same, their Sabbath, for example, began on Saturday morning.