General Catholic
Why do Catholics go to Mass on Sunday instead of the Sabbath?
The apostolic Church shifted its principal worship to Sunday, 'the Lord's Day,' because that was the day of Christ's Resurrection, celebrating the New Covenant inaugurated by the empty tomb rather than merely commemorating the completion of the old creation.
Catholic teaching
Sunday, the 'eighth day,' is kept holy as the memorial of the Resurrection and the day the Church gathers for the Eucharist, fulfilling and surpassing the Sabbath rest by celebrating the new creation begun in Christ.
Biblical foundation
Christ rose 'on the first day of the week' (Mark 16:2), the disciples gathered to 'break bread' on that same day (Acts 20:7), and Paul instructs the Corinthians to set aside collections 'on the first day of every week' (1 Cor 16:2), showing Sunday's early liturgical significance.
Scripture references
Historical evidence
St. Ignatius of Antioch writes around AD 107 that Christians 'no longer observe the Sabbath but live in accordance with the Lord's Day,' and St. Justin Martyr's First Apology (c. 155) describes Christians gathering for Eucharist specifically 'on the day called Sunday.'
The Church Fathers
“On the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read.”