What is forbidden by the third commandment of God? By the third commandment of God all unnecessary servile work on Sunday is forbidden.
Six days shall you do work; in the seventh day is the sabbath, the rest holy to the Lord. (Exodus 31:15)
What is servile work? Servile work is that which requires labor of body rather than of mind.
When is servile work allowed on Sunday? Servile work is allowed on Sunday when the honor of God, our own need, or that of our neighbor requires it.
Further reading: CCC 2184-2188