The fall of the evil angels occurred sometime in the distant past. Though the Bible does not specifically tell us when this happened, there are a couple of clues.
Everything Perfect
When God finished His creation, everything was said to be very good.
God saw everything that He had made, and indeed, it was very good. And there was evening and there was morning, the sixth day (Genesis 1:31).
When God finished creating, "everything" was "very good." If this refers to all God's creation, then this would include the angelic world. If this is the case, then Satan and the evil angels, did not exist before the seventh day of creation as evil beings.
Genesis 3
In the third chapter of Genesis we find Satan, in the form of the serpent, coming to Eve.
Now the serpent was more crafty than any other wild animal that the LORD God had made. He said to the woman, "Did God say, 'You shall not eat from any tree in the garden'?" (Genesis 3:1).
At this time, sin had already entered into the universe. It seems, therefore, that some time between
Genesis 1:31 and
Genesis 3 this angelic rebellion took place and sin entered into the universe.
Restricted To Earth
It could be argued, however, that in Genesis the emphasis on the perfection of creation was limited to the earth - it did not include the unseen realm of the universe. The writer is only interested in telling his audience about the conditions on the earth, not the entire universe. Therefore from the perspective of the writer, no comment is made about sin being elsewhere in the universe. All the writer is saying is that sin had not reached the earth at this time.
Summary
The Bible says there was no sin on the earth until Adam and Eve sinned in the Garden of Eden. This does not necessarily mean there was no sin elsewhere in the universe. If the Genesis creation account is restricted to the earth, then sin could have entered at a previous time somewhere else. If, however, the Bible is saying that everything, everywhere in the universe was perfect, then sin did not occur until after the sixth day of creation.