What is the third commandment of God? The third commandment of God is: Remember thou keep holy the Lord's day.
And on the seventh day God ended his work which he had made; and he rested on the seventh day from all his work which he had done. (Genesis 2:2)
Why does the Church command us to keep Sunday as the Lord's day? The Church commands us to keep Sunday as the Lord's day, because on Sunday Christ rose from the dead, and on Sunday the Holy Ghost descended upon the apostles.
Further reading: CCC 2168-2173