Here is my take on the fall of Satan. The Bible does not state it. You can glean some time when it happened, but not the exact time of it. For one thing Satan being an angel was created way before this creation was, well, created. The angels first abode is in Heaven, the place where God lives which is outside of creation. The angels were already there.
Jde 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
This verse shows that they live in Heaven not creation.
Here in Job, the Lord is talking about when He created creation. As you see the angels were already there.
Job 38:4 Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
Job 38:5 Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
Job 38:6 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
Job 38:7 When the morning stars sang together, and all the sons of God shouted for joy?
So for starters we see that it didn't happen on the first day. In the beginning of Job, we know that he still has access to Heaven.
Job 1:6 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
This happens some time AFTER the Flood. But he is now called Satan and not Lucifer. We know that eventually Satan is cast out from Heaven and stuck inside of Creation.But we do not know when that happens or happened either.
That is a pretty neat answer i know some would think that Satan is of the angelic order but technically he is a cherub.
Ezekiel 28:12 Son of man, take up a lamentation upon the king of Tyrus, and say unto him, Thus saith the Lord GOD; Thou sealest up the sum, full of wisdom, and perfect in beauty.
13 Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created.
14 Thou art the anointed cherub that covereth; and I have set thee so: thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire.
15 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.
16 By the multitude of thy merchandise they have filled the midst of thee with violence, and thou hast sinned: therefore I will cast thee as profane out of the mountain of God: and I will destroy thee, O covering cherub, from the midst of the stones of fire.
17 Thine heart was lifted up because of thy beauty, thou hast corrupted thy wisdom by reason of thy brightness: I will cast thee to the ground, I will lay thee before kings, that they may behold thee.
So sometime before Eden satan fell