Following the "bread and wine" incident of Genesis 14:18, Abraham asks for clarification of his status with God, because earlier, in Genesis 12, God had implied that Abraham's family would be great. After Abraham asks for clarification, God give the promise using an illustration involving stars. In order for Abraham to see stars, this event had to take place at night.
Notice Exodus 12:5-6:
Your [Passover] lamb shall be without blemish, a male of the first year; you shall take it out from the sheep, or from the goats: And you shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening.
This is one of those places where the word "evening" is from the term in Hebrew ben ha arbayim. In modern English it means "twilight" or "dusk." The meaning of this word describes the time that the sun has gone down, but light continues to remain for a period of time. At this time of the year, the light would have lingered very close to about 45 minutes. After that, it would be dark.
Abraham is brought bread and wine by Melchizedek. The next thing we see in Genesis 15 is the mention of "stars"; it is dark. The Passover takes place in that period of dim light before it becomes dark. That is the time that we, in our observance, normally take Passover, just as the sun goes down. That is where the opening of Genesis 15 is time-wise. By the time you see stars, it is dark. We are beginning to see that time is moving in this episode.
When ben ha arbayim takes place, the Abib 13 has ended and Abib 14, Passover day, begins. This is undoubtedly when Melchizedek brought forth the bread and wine. Then came Abraham's vision, when it was dark and the stars were out. It is clearly into Abib 14, because it is dark.
John W. Ritenbaugh
The Wavesheaf and the Selfsame Day
Abraham was concerned that he had no children. In this section, God promised him that he would have countless children. Into this comes faith; Abraham believed Him. It is that simple. When it says that Abraham believed, we can understand from James 2 and elsewhere that his belief motivated him to submit—to obey God.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part Twenty-Seven)