The first day of Unleavened Bread begins at sunset, marking the start of the 15th of Abib or Nisan, following the Passover on the 14th. This beginning aligns with the Israelite tradition of days starting at sunset, as darkness commences. The departure of Israel from Egypt occurred at the beginning of the 15th, under moonlight, not in broad daylight, confirming that the Feast of Unleavened Bread starts at this time. The term "ben ha arbayim," meaning twilight or between the evenings, indicates a specific period immediately following sunset, marking the start of a new day, such as the beginning of the 14th for Passover. In contrast, "ba erev" is a broader term that can refer to a larger part of the day, but in the context of the Days of Unleavened Bread, it designates the end of the 21st of Abib, establishing the boundary for the seven-day observance that begins as the 14th ends and the 15th starts. This clear distinction in timing underscores that the first day of Unleavened Bread commences at the onset of the 15th, separate from the events of Passover on the 14th.

God reveals that His days begin with the evening, which is the night portion, followed by the morning, the daylight portion. This is evident from Genesis 1:5, where God called the light Day, and the darkness He called Night, establishing that the evening and the morning constitute the first day. The Hebrew word translated as "at twilight" in passages related to Passover appears first in Genesis 1:5 as "the evening," confirming that the day begins at evening. Furthermore, sunset marks the start of the evening, which is the beginning of a day in God's perspective.