Matthew Poole and continuators on Exodus 12:18

18 In the first month, on the fourteenth day of the month, at even ye shall eat unleavened bread untill the one and twentieth day of the month at even.

Inclusively. For otherwise they were obliged to eat unleavened bread eight dayes, viz. on the day of the Passe-over, ver. 8. and seven dayes after, which is strictly and properly called the Feast of unleavened Bread, because in them they were tyed to that ceremony onely, except the two days of an Holy Convocation.

Marginal references: Levit. 23. 5. Numb. 28. 16.