Passover / Pesach 2000 years ago ....

Matthew 26:17-18 17

On the first day of the Festival of Unleavened Bread (14th Nissan / March 30), the disciples came to Jesus and asked, "Where do you want us to make preparations for you to eat the Passover?"

He replied, "Go into the city to a certain man and tell him, 'The Teacher says: My appointed time is near. I am going to celebrate the Passover with my disciples at your house.'"

Image Source: http://margmowczko.com/the-covenant-meal/

Biblical Tradition:

Just before the nation of of Israel left Egypt, God commanded them to sacrifice the paschal lamb and then eat it with unleavened matzah and bitter herbs.2 God then told them that they should replicate this feast every year on the anniversary of the Exodus: “It shall be for you a remembrance . . . seven days you shall eat matzah, and on the first day you should remove all se’or (sourdough, a leavening agent) from your homes. Anyone who eats chametz (leaven) from the first day to the seventh day shall be cut off from Israel.”

Chametz (also spelled "hametz" or "chometz") is any food product made from wheat, barley, rye, oats or spelt that has come into contact with water and been allowed to ferment and “rise.”

In practice, just about anything made from these grains—other than Passover matzah, which is carefully controlled to avoid leavening—is to be considered chametz. This includes flour (even before it is mixed with water1), cake, cookies, pasta, breads, and items that have chametz as an ingredient, like malt.

When Is It Forbidden?

According to Jewish law, it is forbidden to eat chametz after the fourth halachic hour 3 on the morning before Passover. It is forbidden to derive any benefit from chametz at the fifth hour, and all chametz should be burned before the sixth hour. From then until after Passover, chametz is completely forbidden.

Passover: https://www.chabad.org/holidays/passover/pesach_cdo/aid/871715/jewish/Passover-Pesach-2018.htm

Why does the prohibition start before Passover begins?

The Torah states: “You shall slaughter the Passover sacrifice to the Lord, your God. . . . You shall not eat leaven with it.”

Tradition interprets this to mean that the prohibition of chametz starts from the time when the Passover sacrifice could be offered: from midday of the 14th of Nissan.

To prevent people from transgressing the prohibition inadvertently, the sages decreed that the prohibition of eating chametz starts two hours before midday, and the prohibition of deriving any benefit starts one hour prior to midday.

Source: https://www.chabad.org/holidays/passover/pesach_cdo/aid/1742/jewish/What-Is-Chametz.htm