Well, it's because Jesus rose from the dead on that day. It's the beginning of the new creation. The early Christians compared the issue with other things in the Mosaic law, such as circumcision.
Read all about it, along with quotes from the early Church, at Catholic Answers in this article about Sunday Worship.
And after you read that article you can "Like" it on Facebook so all of your friends will know. LOL What a wonderful weird world we live in.
|I have a cat. I have a bed. I have a computer. But none of these are mine.|
Side Note: A few Protestant denominations worship on Saturday. But this idea came about much later in Christian history. For instance, the Seventh Day Adventists formed in the middle of the 19th century.