My money's on day 3. Given the record from the only Person I know who was there.
Gen 1:9 And God said, Let the waters under the heaven be gathered together unto one place, and let the dry land appear: and it was so. Gen 1:10 And God called the dry land Earth; and the gathering together of the waters called he Seas: and God saw that it was good. Gen 1:11 And God said, Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself, upon the earth: and it was so. Gen 1:12 And the earth brought forth grass, and herb yielding seed after his kind, and the tree yielding fruit, whose seed was in itself, after his kind: and God saw that it was good. Gen 1:13 And the evening and the morning were the third day.