Satan was created during the six-day period of creation.
We previously established that, in the Bible, the phrase "the heavens" refers to GOD's heavenly creatures. In the beginning, the heavenly creatures also served as stars that illuminated the earth. Sometimes, they Scripture refers to them as "the heavenly hosts," which means the armies of heaven.
1 Alleluia; of Aggeus and Zacharias.
Praise the Lord from the heavens;
Praise Him in the highest.
2 Praise Him, all you His angels;
Praise Him, all you His hosts.
3 Praise Him, sun and moon;
Praise Him, all you stars and light.
4 Praise Him, you heavens of heavens,
And you waters above the heavens.
5 Let them praise the Lord's name,
For He spoke, and they were made;
He commanded, and they were created.
6 He established them forever and unto ages of ages;
He set forth His ordinance, and it shall not pass away.
(Psalm 148:1-6 OSB)
6 And Ezra said, “You are the only Lord. You have made heaven, the heaven of heavens, with all their host, the earth and everything on it, the seas and all that is in them. You give life to all, and the host of heaven worships You.
(Nehemiah 9:6 OSB)
Satan was one of these heavenly beings. In the introduction to the book of Job, Satan is included as one of the "sons of God", which the Greek Old Testament expressly calls "angels."
6 Now there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them.
(Job 1:6 NASB)
6 Then as it so happened one day that behold, the angels of God came to present themselves before the Lord, and the devil also came with them.
(Job 1:6 OSB)
Based on the Genesis account, as one of these heavenly creatures, Satan would have come into existence on day 4 of the creation week:
14 Then God said, “Let there be lights in the firmament of heaven for illumination to divide day from night. 15 Let them be for signs and seasons, and for days and years. Let them be for illumination in the firmament of heaven to give light on the earth.” It was so. 16 Then God made two great lights: the greater light to rule the day, and the lesser light to rule the night. He made the stars also. 17 God set them in the firmament of heaven to give light on the earth, 18 and to rule over the day and over the night, and to divide the light from the darkness. God saw that it was good. 19 So evening and morning were the fourth day.
(Genesis 1:14-19 OSB)
Illumination here has multiple meanings: physical light and life guidance.
Later, as GOD was giving Moses the ten commandments of the Law, He expressly stated that everything which was created, both in heaven and on earth, was made during the creation week:
11 For in six days the Lord made heaven and earth, the sea, and everything in them, and rested on the seventh day.
(Exodus 20:11a OSB)
Therefore, contrary to what many claim, Satan did not exist before the creation week. He was not present before the earth was formed. As a heavenly creature, Satan apparently came into existence on day 4 of creation.