Holy Gospel of Jesus Christ according to Saint Luke 6:1-5.
While Jesus was going through a field of grain on a sabbath, his disciples were picking the heads of grain, rubbing them in their hands, and eating them.
Some Pharisees said, "Why are you doing what is unlawful on the sabbath?"
Jesus said to them in reply, "Have you not read what David did when he and those (who were) with him were hungry?
(How) he went into the house of God, took the bread of offering, which only the priests could lawfully eat, ate of it, and shared it with his companions."
Then he said to them, "The Son of Man is lord of the sabbath."
We don't really know when is exactly the day of the Sabbath. When we talk about the sabbath day, we just follow that on the seventh day of the creation God take a rest. So, we follow that we should take our rest and make the seventh day our rest and holy because God also take a rest on that day. However, who among us knows the exact day of the sabbath which is the seventh day? Remember, that God created the world without a calendar. So, to my mind, when we talk about the sabbath we must follow our conscience. It's up to us what particular day that we will make it a sabbath day. It maybe Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, or Sunday. But since our Catholic faith told us that we must make Sunday as the day of the Sabbath, then we shall follow it, as Sunday is almost all Catholic's Sabbath day. However, if you are working on Sundays, you can always designate what particular day you wanted to make it your sabbath day. In the end Jesus said "The Son of Man is lord of the sabbath".