The odd practice of starting a holiday a half a day early
The Bible requires the Israelites to observe Passover on the fourteenth day of the first month, currently called Nissan, by sacrificing a lamb, called the Pascal Lamb, and eating it together with unleavened bread and bitter herbs. This biblical holiday is followed immediately with a seven day holiday called The Festival of Unleavened Bread. When the temple was destroyed in 70 CE, sacrifices were discontinued and since the sole practice of Passover was the sacrificing and eating of the Pascal Lamb, the holiday of Passover ceased to exist. So that the memory of Passover would not disappear entirely, the holiday of The Feast of the Unleavened Bread received the additional name of Passover.
Although not mentioned in the Torah, a practice arose to cease eating leavened practices around noon on the day before The Feast of Unleavened Bread. Why did this practice arise? I do not know the reason for this unusual behavior of beginning a key element of a holiday before the holiday starts. However, I have a suggestion to explain the anomaly.
We should recall that (1) the elements of the biblical Passover are the sacrifice and eating it with unleavened bread and bitter herbs, (2) although the biblical Passover ceased to exist, Jews felt that they should remember the holiday and did so by adding a name to The Festival of Unleavened Bread, (3) the Torah requires that the Pascal sacrifice be brought at mid-day on the fourteenth of the first month, and (4) the most significant element of the two holidays is unleavened bread, eating it with the Pascal Sacrifice on the biblical Passover and abstention from eating leavened bread during The Festival of Unleavened Bread. It is possible that the tradition to abstain from leavened bread during mid-day on the fourteenth, during the time that the Pascal Lamb was brought, was instituted, just as the adding of the name Passover to The Festival of Unleavened Bread, to recall the holiday and practice of the biblical Passover.