My favorite part was this paragraph:
The reason for the Sabbath change is straightforward. The Sabbath was a sign of the Old Covenant. But Jesus has brought a New Covenant and a New Priesthood. So the Church began observing the Sabbath on the day of his resurrection, which established that covenant. You can already see it reflected in Revelation, when John says, "I was in the Spirit on the Lord's Day" (meaning "I was at Mass. It was Sunday").
But please, go read the rest and get the full explanation, as well as a nifty link to even more information on the New Covenant's Sabbath.