Here's something to ponder. In Exodus 12:1 it clearly says:
And the Lord spake unto Moses and Aaron in the land of Egypt, saying, This month shall be unto you the beginning of months:it shall be the first month of the year to you
The first day of the Passover, is to be the first day of the first month of the year. In other words, the year is to begin on the day determined to be the first of Nisan, not the day on which they celebrate the New Year, Rosh Hashanah. In fact the Torah does not mention Rosh Hashanah. The reason for it is the same reason that the new school year in the north starts in September and not in January as it does in the south: because of the harvest.
Sometime between the Torah and the codification of the Mishnah, the autumn new year gained ascendance, now transformed into a major celebration, and the Nisan new year was left as a marker of the months and festivals in the calendar year. Although theories abound about the causes of this transition, the mechanics are lost in the web of historical change. The Talmudic rabbis analyze the text of the Bible as they argue about when the New Year should began, yet different sets of verses yield different answers. Historians cite evidence from the ancient Near East, looking at the new years celebrated by neighboring peoples, but nothing is conclusive. Others look to archeology for support. But the truth remains murky.
Some ancient Semitic peoples considered the year to begin around the autumn harvest and the beginning of the rainy season, which both signified the start of a new agricultural year. Although the Torah never explicitly refers to an autumn new year, some scholars see in the Torah’s apparent timing of the fall harvest festival (Sukkot) a small hint of a possible fall new year. According to Exodus 23:26, the Feast of the Harvest, which closely follows Rosh Hashanah and Yom Kippur, occurs, b’tzayt ha-shanah, at the going out of the year, signifying the close of one agricultural year and the beginning of the next. Similarly in Exodus 34:22, the Feast of the Ingathering is said to occur t’kufat hashanah, “at the turn of the year.” Further evidence of the fall as the beginning of the agricultural year in Palestine is a calendar from the 10th century BCE found at Tel Gezer, which begins with the two Months of the Ingathering.
http://www.myjewishlearning.com/article/how-rosh-hashanah-became-new-years-day/2/So it seems that not only Christians cherry-pick which verses of the Bible they choose to obey.
Here's some more: it says later on that 600,000 men left Egypt. That's a huge number of people. Considering that maybe not all of them had sheep, so they had a lamb to slaughter, or didn't have a big enough household to justify the killing of a lamb for only one or two people, so they went to a neighbour's house to celebrate. How did the Egyptians not notice more than half a million lambs being slaughtered and barbecued on the same day? Imagine if your neighbours on either side of you decided to hold a barbecue cooking meat of that size, a whole spit-roasted lamb which would take around a day to cook, wouldn't you wonder why they were doing that? But not only that, why were they also painting their doorways with the blood? And why didn't an all-knowing god know which houses were holding a barbecue, did he also lose his sense of smell? Or was he not yet omniscient?
A mind without instruction can no more bear fruit than can a field, however fertile, without cultivation. - Marcus Tullius Cicero (106 BCE - 43 BCE)