Charles Ellicott 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. —The Hebrew omits “month” by a not unusual ellipsis. (Compare to Ezekiel 1:1.)

At even. —The evening intended is not that with which the fourteenth day began, but that with which it closed, the end of the fourteenth and beginning of the fifteenth day. (See Leviticus 23:5-6.)