John Gill Commentary Exodus 12:18

"In the first [month], on the fourteenth day of the month at even, ye shall eat unleavened bread, until the one and twentieth day of the month at even." — Exodus 12:18 (ASV)

In the first month As it was now ordered to be reckoned, the month Abib or Nisan: the fourteenth day of the month at even, you shall eat unleavened bread; that is, at the evening following, the fourteenth of Nisan, and which was the beginning of the fifteenth day, the Jews beginning their day from the evening: hence the Targum of Jonathan is,

"on the fourteenth of Nisan you shall slay the passover, in the evening of the fifteenth you shall eat unleavened bread:"

until the twentieth day of the month at even; which would make just seven days; the above Targum adds, "on the evening of the twenty second you shall eat leavened bread," which was the evening following the twenty first day.

This long abstinence from leaven denotes, that the whole lives of those who are Israelites indeed should be without guile, hypocrisy, and malice, and should be spent in sincerity and truth.