This is a question that I hear frequently when we talk about the third commandment (if you forgot it's: "Remember the Sabbath Day to keep it holy").

The "Sabbath Day" (literally "day of rest") was the seventh day of the week -the day we know as "Saturday."

God was very adamant in his command that everyone set aside the seventh day to "recharge their physical batteries" by resting from their labors and to "recharge their spiritual batteries" by gathering together with the other members of the family of God for worship:

The Sabbath was to be the seventh day, following the example of God when he rested on the seventh day after creating the world the six days previous.

Quite simply because Sunday is the day of The Resurrection, the day Jesus rose from the grave.

The Resurrection is the central event of history:

Given all that The Resurrection means to our faith, it was only natural that the first Christians began meeting on Sunday in celebration of what happened on that day. Just as the Sabbath (Saturday) was a commemoration of the Lord's work in Creation, Sunday became a commemoration of the New Creation brought by the forgiveness of the sins that ruined creation.

Even though Christians began to gather to celebrate the Resurrection each Sunday, they also continued to observe the Sabbath (see Acts 13-18).

It's not clear exactly when Sunday became the sole day of worship and rest for Christians, but this likely happened when it became necessary to differentiate between Jews who accepted Jesus as Messiah and Lord and those who didn't.

The Christian church has always viewed each Sunday worship as a "mini-Easter." In other words, a time to celebrate, not a somber time, but a joyous, praise-filled time.

It is for this reason that the Sundays which occur during the Lenten season (a somber season void of celebration) are actually not considered a part of the season. Joyous "alleluias" which are not voiced during Lent, are still appropriate for the Sundays that fall in Lent, because each and every Sunday is a "mini-Easter" celebration designed to remind us that "in Christ we are a new creation - the old has gone, the new has come."

So are we still breaking the third commandment by worshipping on Sunday and not Saturday? No.

The explanations on pages 66-67 of Luther's Small Catechism (1991 version) say it well: The Sabbath was a sign pointing to Jesus (and the eternal rest purchased by his death and guaranteed by his resurrection). Since Jesus has now come as our Savior and Lord, we are no longer bound to follow the sign that pointed to his coming (the Sabbath).

Although we are not bound to worship on Saturday, God still does require us to gather together for worship (see Hebrews 10:25). What better day to do it than on the day Jesus rose from the grave?!