Sunday is not the Sabbath, Sunday is the day that most Christians set aside to worship the Lord. Sunday is the first day of the week on every calendar I have seen.
In the Ten Commandments the Sabbath is called a day of rest; in Leviticus the Sabbath is a holy convocation for the Jews. Now the issue seems to be are we under the Law, and the answer is no.
There are a plethora of interesting sites on the Internet but that does not mean those interesting sites are Biblically based or Biblically sound. If I took the time to correspond with every author of every Internet site that posts an opinon that contradicts Scripture I would not have time to live a productive life. I am not going to contact the author of the site you posted.