What are we commanded by the third commandment? By the third commandment we are commanded to worship God in a special manner on Sunday, the Lord's day.
Keep you my sabbath; for it is holy unto you. (Exodus 31:14)
How does the Church command us to worship God on Sunday? The Church commands us to worship God on Sunday by assisting at the Holy Sacrifice of the Mass.
Further reading: CCC 2174-2183