When the remnant of the house of Judah returned from Babylon they brought back corruption. That is where Phariseeism began. One corruption was the naming of the months. This included the figuring of days, times and events such as Passover based on the moon. Yahweh God’s first month of the year is Abib.
Deuteronomy 16:1 Observe the month of Abib, and keep the Passover to Yahweh your God; for in the month of Abib Yahweh your God brought you forth out of Egypt by night.
Nissan was the first month of the year in Babylon. When Israel lived in Babylon they had to go by the local calendar, To bring that calendar back with them was a violation of Yahweh God’s command. God’s calendar is based on the Sun. The day began with light. Babylon and the Pharisees used a calendar based on the moon. The day began with darkness at sunset. God’s calendar is simple and easy for anyone to follow. The Babylonian calendar is complex and can sometimes have 13 months in a year. This is why you find phrases such as:
John 2:13 “The Passover of the Judeans (Jews) was at hand, and Jesus went up to Jerusalem.”
It is the difference between light and darkness, day and night, life and liberty in God or sin and death in outer darkness.