According to the Bible, the Sabbath day is Saturday. So why do Catholics not celebrate Sunday as Sabbath? Where in the Bible can I find a reference to Sunday being the Sabbath day.

As you have rightly said, Sabbath according to the Bible is on Saturdays. The Church did not move Sabbath from Saturday to Sunday or to any other day. Instead “The Sabbath, which represented the completion of the first creation, has been replaced by Sunday, which recalls the new creation inaugurated by the Resurrection of Christ” (CCC 2190).

We are no more interested in Sabbath, instead, we are celebrating the Lord’s day which is on Sunday. Sabbath according to Jewish law is the last day of the week (Saturday), when God rested from all the work he had done in creation. Whereas the Lord’s Day is the first day of the week (Sunday); the day when God said “Let there be light”; the day when Christ rose from the dead; the day when the Holy Spirit came upon the Apostles. 

“The Church celebrates the day of Christ’s Resurrection on the ‘eighth day,’ Sunday, which is rightly called the Lord’s Day” (CCC 2191).

We can see in the New Testament that the early fathers were celebrating Lord’s day and not Sabbath. They were denouncing Sabbath and were urging the Catholics to celebrate the Lord’s day.

On the first day of the week when we gathered to break bread, Paul spoke to them because he was going to leave on the next day, and he kept on speaking until midnight (Acts 20:7).

On the first day of the week, each of you should set aside and save whatever one can afford, so that collections will not be going on when I come (1 Cor. 16:2)

Let no one, then, pass judgment on you in matters of food and drink or with regard to a festival or new moon or Sabbath (Col. 2:16).