Twilight

God called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day (Gen 1:5). 

The sequence of dusk followed by dawn is puzzling to commentators. Sarna thinks it reflects the Jewish calendar, where Passover and other festivals begin and end at sunset. 

Dusk to dawn is a way of saying "night". The literary function may be to transition from one day to the next. 

Sunlight is a positive theological metaphor. That's because humans are diurnal creatures who rely on eyesight. If we were nocturnal creatures, then sunlight wouldn't have the same emblematic significance.

The syntactical relation between v1 and v2 is disputed. If v1 is included in the events of the first day, then there was, in the absolute sense, a first day. 

Because sunup and sundown are cyclical–indeed, paradigmatic examples of periodic phenomena–it seems like there's always another day before today. But according to one reading of Gen 1, if you step into the time machine, you can only go back as far as the first day. There's no day before that. Nothing before that. No time or space. There's God, but the time machine won't take you to God.

Twilight is ambiguous. Is it the beginning of a new day or the ending of the day? Will it get brighter or darker? If you woke up outside at twilight, could you tell if it was dawn or dusk?

If you had a sense of direction, you could infer that from the direction of the light. Of course, the significance of east and west in that regard is something we appreciate from experience. But if, like Adam, on the day he was made, we had no prior experience of the sun rising in the east and setting in the west, we couldn't detect or predict what twilight represented. To experience the new world for the first time would be bewildering, in a nice way.