Q. How can the Kiddush say that on the 7th day God finished His work? Surely He rested on that day.
A. The Kiddush is quoting from Gen. 2:2, which does seem to say that God was working on Shabbat. Rashi offers two explanations:
1. It is only an apparent difficulty; only to our eyes does it seem that He was still working on Shabbat, but in fact He finished at the exact moment when the 6th day ended and the 7th began.
2. What God did on the 7th day was to bring rest into being (“What did the world lack? Rest. Sabbath came; rest came”). Ibn Ezra says that the completion of an action is not in itself an action; hence, when the 7th day began, God’s work was already completed. Cassuto understands “and He finished” in the pluperfect, i.e. “On the 7th day God had finished His work…”.