Why do some translations of Genesis 2:2 say that God finished his work on the sixth day, and others on the seventh day?

This text is from a letter written by David Gooding in 1999.

Some translations may be following the Septuagint rather than the Masoretic text. The Masoretic text of Genesis 2:2–3 says that God rested on the seventh day:

and on the seventh day, God finished his work which he had made, and he rested on the seventh day from all his work which he had made. And God blessed the seventh day and hallowed it, because that in it he rested from all his work which he had created and made.

The Septuagint translators seem to have argued that, since God rested on the seventh day and did no work on that day, he must have ceased from his work on the sixth day. So they have it that God ceased from his work on the sixth day.

This question was much debated among the ancient rabbis, but the Masoretic text is correct.