It seems to me that you would find any mention of the first day of the week and anything that happened on that day is used to support the idea of worshiping on Sunday. Mainstream Christianity worships on Sunday only because the Roman Catholic Church and Emperor Constantine changed the day of worship to Sunday. The reason for your keeping Sunday as your day of worship is simply tradition and not a command of God.
In answering this point, we need to note that Constantine reigned from 306 to 337 AD. You claim that the day of worship changed by a decree of Constantine; hence, we should find that Christians worshiped on the Sabbath prior to Constantine and on Sunday after Constantine. However, history does not support this contention.
As noted elsewhere, we know that the early disciples worshiped on the first day of the week. "Now on the first day of the week, when the disciples came together to break bread, Paul, ready to depart the next day, spoke to them and continued his message until midnight" (Acts 20:7). "On the first day of the week let each one of you lay something aside, storing up as he may prosper, that there be no collections when I come" (I Corinthians 16:2). However, these are not the only statements made by early Christians. There is a large body of writings done by early Christians. While these writings are not inspired, they do give us insight into the beliefs and practices of early Christians.
Didache, between 80 and 140 AD
"But every Lord’s Day, gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, so that your sacrifice may be pure.”
Ignatius, about 107 AD
“If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again in Him...Let us therefore no longer keep the Sabbath after the Jewish manner, and rejoice in the days of idleness; for "he that does not work, let him not eat." ...let every friend of Christ keep the Lord's day as a festival, the resurrection day, the queen and chief of all the days [of the week]”
Aristides, about 125 AD
“However, [the Jews,] too have erred from true knowledge. In their imagination, they think that it is God whom they serve. Actually, by their type of worship, they render their service to the angels and not to God. For example, they do this when they celebrate Sabbaths.”
Justin Martyr, about 160 AD
“Is there any other matter my [Jewish] friends, in which we Christians are blamed, than this: that we do not live after the Law ... and do not observe Sabbaths, as you do?”
“There was no need of circumcision before Abraham. Nor was there need of the observance of Sabbaths, or of feasts and sacrifices, before Moses. Accordingly, there is no more need of them now.”
“And 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, as long as time permits. Then, when the reader has ceased, the president verbally instructs us and exhorts us to imitate these good things. Then we all rise together and pray. And, as we said before, when our prayer is ended, bread and wine and water are brought. Then, the president in like manner offers prayers and thanksgivings, according to his ability. And the people assent, saying “Amen.” Then, [the Lord’s Supper] is distributed to everyone, and everyone participates in [the bread and wine], over which thanks has been given. And a portion of it is sent by the deacons to those who are absent. ... But Sunday is the day on which we all hold our common assembly, because it is the first day on which God ... made the world. And Jesus Christ our Savior rose from the dead on that same day.”
Eusebius, citing Irenaeus, about 180 AD
"This custom of not bending the knee on Sunday is a symbol of the resurrection, through which we have been set free by the grace of Christ.”
Tertullian, about 197 AD
“We devote Sunday to rejoicing for a far different reason than sun worship. ... Others ... suppose that the sun is the god of the Christians ... because we make Sunday a day of festivity.”
“Just as the abolition of fleshly circumcision and of the old Law is demonstrated as having been consummated at its specific times, so also the observance of the Sabbath is demonstrated to have been temporary.”
Anatolius, about 270 AD
“It should not be lawful to celebrate the Lord’s mystery of Easter [i.e. the Lord’s Supper] at any other time but on the Lord’s Day, the day on which the Lord’s resurrection from death took place.”
Victorinus, about 280 AD
“And let this become a rigorous fast, lest we should appear to observe any Sabbath with the Jews. For concerning [their Sabbath], Christ himself, the Lord of the Sabbath, says by His prophets that “His soul hates.” In His body, He abolished this Sabbath.”
The evidence is historically solid that early Christians, long before Constantine, worshiped on the first day of the week and not on the seventh day. Constantine could not have instituted a new practice since the practice predates him.
However, I was curious what Constantine decreed that cause people like yourself to think that he changed the day of worship. What I found was that on March 7, 321 AD Constantine issued this decree: “On the venerable Day of the Sun let the magistrates and people residing in cities rest, and let all workshops be closed. In the country, however, persons engaged in agriculture may freely and lawfully continue their pursuits.” The decree doesn't mention anything in regards to activities of the church. Essentially it is the first "blue Sunday" law or "Sunday closure" law. The only reason a Sabbatarian would think that this dealt with worship is because the Jewish Sabbath day was celebrated by resting. Christians do not worship by resting because our rest comes later. "There remains therefore a rest for the people of God. For he who has entered His rest has himself also ceased from his works as God did from His. Let us therefore be diligent to enter that rest, lest anyone fall according to the same example of disobedience" (Hebrews 4:9-11). In addition, Constantine's decree only applied to city dwellers; farmers were specifically exempted from the law.