Genesis 2:4 These are the generations of the heavens and of the earth when they were created, in the day that the Lord God made the earth and the heavens,
Genesis 1:1 In the beginning God created the heaven and the earth.
So it happened in one ("yom"). But "yom" doesn't have to mean "day." It can mean "always" "forever" "some unspecified amount of time", "in my time" and so on. Clearly Genesis, by saying creation was six yom in one chapter and saying it was in one yom in the next chapter, makes it clear that "yom" was not used as a period of time.