Moses 2

According to the revelation given to Moses, on the very first day -

"...God, said: Let there be light; and there was light.
4 And I, God, saw the light; and that light was good. And I, God, divided the light from the darkness.
5 And I, God, called the light Day; and the darkness, I called Night; and this I did by the word of my power, and it was done as I spake; and the evening and the morning were the first day."  (Pearl of Great Price | Moses 2:3 - 5)

It becomes obvious upon further reading that this light was not the light of the Sun or the Moon.  In my mind, this light was the Light of Christ which light "is good" or goodness or truth -

"And the light which shineth, which giveth you light, is through him who enlighteneth your eyes, which is the same light that quickeneth your understandings;
12 Which light proceedeth forth from the presence of God to fill the immensity of space—"  (Doctrine and Covenants | Section 88:11 - 12)

Which also is -

"...The light which is in all things, which giveth life to all things, which is the law by which all things are governed, even the power of God who sitteth upon his throne, who is in the bosom of eternity, who is in the midst of all things."  (Doctrine and Covenants | Section 88:13)

This light must have been in place on the first day to give life and govern all things including that which followed in the creation including the firmament and solar system for -

"...This is the light of Christ. As also he is in the sun, and the light of the sun, and the power thereof by which it was made.
8 As also he is in the moon, and is the light of the moon, and the power thereof by which it was made;
9 As also the light of the stars, and the power thereof by which they were made..."  (Doctrine and Covenants | Section 88:7 - 9)

The light was called "day' because -

"That which is of God is light; and he that receiveth light, and continueth in God, receiveth more light; and that light groweth brighter and brighter until the perfect day."  (Doctrine and Covenants | Section 50:24)

It was given in the first day becasue -

"...the Spirit giveth light to every man that cometh into the world; and the Spirit enlighteneth every man through the world, that hearkeneth to the voice of the Spirit."  (Doctrine and Covenants | Section 84:46)

And the absence of receiving such light is darkness or loss of light or truth for -

"...I say it that you may know the truth, that you may chase darkness from among you..."  (Doctrine and Covenants | Section 50:25)

Amulek explained night and darkeness as -

"...I beseech of you that ye do not procrastinate the day of your repentance until the end; for after this day of life, which is given us to prepare for eternity, behold, if we do not improve our time while in this life, then cometh the night of darkness wherein there can be no labor performed."  (Book of Mormon | Alma 34:33)

Note that when there is the absence of the Light of Christ, it becomes night and darkness prevails.