Q. What was the reason the Sabbath was changed to Sunday?
The reason for the change is that Jesus Christ rose from the dead on a Sunday morning. That is why those who believe in him have made Sunday their sacred day of the week.
I should specify that the Sabbath itself is still on Saturday. The New Testament writers, who were among the earliest followers of Jesus, instead call Sunday “the Lord’s Day.” John says at the beginning of the book of Revelation, “I was in the Spirit on the Lord’s Day,” meaning on a Sunday or the first day of the week. Various Christian communities have continued to use this language, for example, by offering “Lord’s Day School” rather than “Sunday School.” When New Testament writers refer to the Sabbath, they mean Saturday. The Spanish language still expresses this distinction; it calls Saturday el sábado (“the Sabbath”) and Sunday domingo (a shortened form of “the Lord’s Day”).
Many early Christian communities were composed of a mixture of Jews who had come to believe in Jesus as their Messiah, non-Jews who had been worshiping in the synagogue and following some Jewish practices, and non-Jews who did not have that background. So there was a question in those communities about whether to observe Saturday as a Sabbath day of rest and no work. In his letters, the apostle Paul counseled believers that this was now a matter of individual conviction: “One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind.” In other words, the Sabbath principle remains one that believers should honor, but they may do that either by setting aside one day a week for rest, refreshment, and worship or by being careful to ensure that they alternate work with rest, refreshment, and worship throughout the week.
It is true that some Christian communities have encouraged their members to set aside Sunday as a day of rest, and as a result, they have referred to Sunday as the Sabbath. If that is the language of your community, this is the reason why they are using it.
Good Question 