Actually, Paul didn’t speak of believers assembling on Sunday several times. The apostles preached every day, so it is not unusual at all that they would preach on a Sunday (or any other day of the week). That is simply an erroneous notion, and those passages cannot be used as proof texts for forsaking the Sabbath, which is something YHWH blessed forever.
The first day of the week WAS set apart ever since the resurrection of Christ.
Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.(Mark 16:9
And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight. (Acts 20:7)
Upon the first day of the week let every one of you lay by him in store, as God hath prospered him, that there be no gatherings when I come. (I Cor. 16:2)
Jesus spoke of the Jewish Sabbath:
And he said unto them, The sabbath was made for man, and not man for the sabbath. (Mark 2:27)
Which goes along with Paul's admonition:
Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ. (Col. 2:16,17)
Who art thou that judgest another man's servant? to his own master he standeth or falleth. Yea, he shall be holden up: for God is able to make him stand.
One man esteemeth one day above another: another esteemeth every day alike. Let every man be fully persuaded in his own mind.
He that regardeth the day, regardeth it unto the Lord; and he that regardeth not the day, to the Lord he doth not regard it. He that eateth, eateth to the Lord, for he giveth God thanks; and he that eateth not, to the Lord he eateth not, and giveth God thanks. (Romans 14:4-6)
It is because of these Scriptures in addition to the actions of the early church under the leadership of the Apostles and their direct disciples that Sunday is generally observed as the day for worship and fellowship. We are no longer under the law - nor were we ever who are Gentiles. The New Testament seems pretty clear that the "day of rest" is between each person and the Lord and we are also admonished:
And let us consider one another to provoke unto love and to good works: Not forsaking the assembling of ourselves together, as the manner of some is; but exhorting one another: and so much the more, as ye see the day approaching. (Hebrews 10:24-25)
Those who would be "Judaizers" (live according to Jewish customs) to the Christians of today, should remember that we are no longer under the law but under grace. Things work differently now.