Why do we celebrate Sunday as the Sabbath when the Jewish people celebrate Saturday as the Sabbath?
We don’t. Sabbath is on Saturday. Catholics worship on Sunday because it is the Lord’s Day, the day on which Christ rose from the dead and created the world anew. In the past, Christians have called Sunday, their day of corporate worship, “the Sabbath” in a colloquial sense to indicate that Sunday is a day of rest for Christians; but this has proved to be an enormous mistake, causing a great deal of confusion. One consequence, for example, was the idea that the Church “moved” the Sabbath from Saturday to Sunday, which led some Protestant groups
– which, since they are Protestant and reject the Church’s authority – to reject the historic Christian understanding of Sunday as the day of Christian worship and to claim that Christians must worship on Saturday (e.g., Seventh-Day Adventists).