Rashi
A man of flesh and blood, who cannot judge his times and his moments, must add some time from the week to Shabbos. G-d, who can judge His times and His moments, entered a hairsbreadth into Shabbos, and it appeared as if He completed the work on that day.
Another explanation: What was the world lacking? Rest. When Shabbos came, rest came. The work was completed and finished.
