From Hagan’s “Year of the Passover.” The historian Josephus was a Second Temple priest, so the dates for the Passover celebration he describes are the ones followed by Jesus and his later chroniclers. Without question.
The Passover celebration commemorates the release of the Hebrew slaves by the Egyptian pharaoh, which occurred an estimated thousand years before the times of Jesus. Note in the following passages that Nisan is described as being the first month in the Jewish Calendar, thus making the first day of Nisan the Jewish New Year as well.
“But when God had signified, that with one plague he would compel the Egyptians to let Hebrews go, he commanded Moses to tell the people that they should have a sacrifice ready, and they should prepare themselves on the tenth day of the month Xanthicus, against the fourteenth, (which month is called by the Egyptians Pharmuth, Nisan by the Hebrews; but the Macedonians call it Xanthicus,) and that he should carry the Hebrews with all they had. Accordingly, he having got the Hebrews ready for their departure, and having sorted the people into tribes, he kept them together in one place: but when the fourteenth day was come, and all were ready to depart they offered the sacrifice, and purified their houses with the blood, using bunches of hyssop for that purpose; and when they had supped, they burnt the remainder of the flesh, as just ready to depart. Whence it is that we do still offer this sacrifice in like manner to this day, and call this festival Pascha which signifies the feast of the passover; because on that day God passed us over, and sent the plague upon the Egyptians; for the destruction of the first-born came upon the Egyptians that night, so that many of the Egyptians who lived near the king’s palace, persuaded Pharaoh to let the Hebrews go. Accordingly he called for Moses, and bid them be gone; as supposing, that if once the Hebrews were gone out of the country, Egypt should be freed from its miseries. They also honored the Hebrews with gifts; some, in order to get them to depart quickly, and others on account of their neighborhood, and the friendship they had with them. (Antiq II 14:6)”
In the month of Xanthicus, which is by us called Nisan, and is the beginning of our year, on the fourteenth day of the lunar month, when the sun is in Aries, (for in this month it was that we were delivered from bondage under the Egyptians,) the law ordained that we should every year slay that sacrifice which I before told you we slew when we came out of Egypt, and which was called the Passover; and so we do celebrate this passover in companies, leaving nothing of what we sacrifice till theday following. The feast of unleavened bread succeeds that of the passover, and falls on the fifteenth day of the month, and continues seven days, wherein they feed on unleavened bread; on every one of which days two bulls are killed, and one ram, and seven lambs. Now these lambs are entirely burnt, besides the kid of the goats which is added to all the rest, for sins; for it is intended as a feast for the priest on every one of those days. But on the second day of unleavened bread, which is the sixteenth day of the month, they first partake of the fruits of the earth, for before that day they do not touch them. And while they suppose it proper to honor God, from whom they obtain this plentiful provision, in the first place, they offer the first-fruits of their barley, and that in the manner following: They take a handful of the ears, and dry them, then beat them small, and purge the barley from the bran; they then bring one tenth deal to the altar, to God; and, casting one handful of it upon the fire, they leave the rest for the use of the priest. And after this it is that they may publicly or privately reap their harvest. They also at this participation of the first-fruits of the earth, sacrifice a lamb, as a burnt-offering to God. (Antiq III 10:5)"
“And as the feast of unleavened bread was at hand, in the first month, which, according to the Macedonians, is called Xanthicus, but according to us Nisan, all the people ran together out of the villages to the city, and celebrated the festival, having purified themselves, with their wives and children, according to the law of their country; and they offered the sacrifice which was called the Passover, on the fourteenth day of the same month, and feasted seven days, and spared for no cost, but offered whole burnt-offerings to God, and performed sacrifices of thanksgiving, because God had led them again to the land of their fathers, and to the laws thereto belonging, and had rendered the mind of the king of Persia favorable to them. (Antiq XI 4:8)”
The Paschal feast of the Passover was celebrated on the fourteenth day of Nisan, with the feast of the unleavened bread being celebrated on the following seven days from the fifteenth through the twenty first days of Nisan.