It doesn't mention the sun in the definition of night and day there. It says "morning and evening." All you need from that is a source of light. I suppose you're wondering how there can be light without a sun. Go back a verse and read, God said, "Let there be light."
Revelation 21:23 KJV
(23) And the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof.
God can provide light before he creates suns and stars, can't he? Even the face of Moses provided light at one time.
Exodus 34:29-30 KJV
(29) And it came to pass, when Moses came down from mount Sinai with the two tables of testimony in Moses' hand, when he came down from the mount, that Moses wist not that the skin of his face shone while he talked with him.
(30) And when Aaron and all the children of Israel saw Moses, behold, the skin of his face shone; and they were afraid to come nigh him.
If you're asking where the light came from, it came from the source, that is, God.