the first day. The eating of the Passover took place on the fourteenth of Nisan. See Exodus 12:6; Exodus 12:8; Exodus 12:18; Leviticus 23:5.Numbers 9:3; Numbers 28:16. The fifteenth was the high sabbath, the first day of the feast. See Numbers 28:17.
Where... ? This question shows that the date was the fourteenth of Nisan.