12:1And a great sign appeared in heaven: a woman clothed with the sun, and the moon under her feet, and on her head a crown of twelve stars;

And a GREAT SIGN appeared in HEAVEN:

Before chapter 12, not much about the conflict with the devil has been recorded. There was a bit about evil angels (demons) in the fifth trumpet (Revelation 9:1ff), but not about Satan's conflict with God. Now we will see it.

The use of "GREAT SIGN" indicates that what John is describing is symbolic and thus chronological ordering may not be inferred here. This is the first of four "GREAT"s in this chapter:

GREAT SIGN verse 1
GREAT RED DRAGON verses 3,9
GREAT WRATH verse 12
GREAT EAGLE verse 14

The use of "GREAT" implies this is larger than individual activity; it is more on the scale of a group/nation (even whole earth). What we are seeing here in the first nine verses of chapter 12 is a history lesson of the earth from God's spiritual perspective. It, obviously, is not the whole perspective but a mere glimpse.

a WOMAN

The female comes out of male and is not original creation. Hence, this WOMAN is not the original but out of someone else (Genesis 2:21-23)