There are those who would point to Leviticus 23:6 and insist that Abib 15 is the first day of the Festival of Unleavened Bread which continues through Abib 21.
And on the fifteenth day of the same month is a festival of unleavened bread unto Yahweh: seven days you must eat unleavened bread. (Leviticus 23:6)
An additional reference is also given from Numbers:
And on the fifteenth day of this month is a festival; seven days shall unleavened bread be eaten. (Numbers 28:17)
For those who observe a form of the Pharisaic/Hasidic practice for the Festival of Unleavened Bread (System B and System G), these passages from Leviticus and Numbers are held as proof that the first day of the 7-day Festival of Unleavened Bread is Abib 15.
It is evident that when these two passages are closely examined in context, the results reveal the opposite and actually support Passover Day, Abib 14, as being the first day of the Festival of Unleavened Bread (System A).