Sunday is the first day of the week when Jesus rose from the grave. It is “the Lord’s Day” but not the Sabbath. The Sabbath was Law and we do not live under the Law. We quit the Sabbath for the same reasons we eat pork, shellfish and wear 50/50 cotton/poly blend fabrics. The early Christians continued going to the Temple/synagogues but also met DAILY in the homes of other believers. Worship daily and you don’t have to worry about which day is the right one. The New Testament command is to not avoid assembling together.
Sabbath keeping is a legal requirement similar to tithing. New Testament Christians are not “required” to tithe. We are to be cheerful givers and honor God with all we have (not just money but time, resources, our house, our car, etc). It is biblical to support your pastor, feed hungry people, send out missionaries and so forth. But those collections are gifts; if you “pay” out of a sense of obligation you might be better off keeping that money. Salvation is a gift that cannot be bought or earned. “Freely you have received, freely give” –Matthew 10:8