Under Mosaic Law the sacred day of the week was the seventh day, which was known as the Sabbath. It was a day of rest during which no work was to be done (Exodus 20:9-10; Deuteronomy 5:14). According to Mosaic Law, the seventh day was to be held in high honor. The fourth command of the Ten Commandments says, “Remember the Sabbath day, to keep it holy” (Exodus 20:8). The reason this day was so sacred is because it correlated to the day of God’s rest following creation (Exodus 20:11), and because it served as a reminder that God freed the Israelites from slavery in Egypt (Deuteronomy 5:15).
A change in the sacred day of the week occurred between the Old Testament and the New Testament. Throughout the New Testament a pattern emerges of congregational assemblies occurring on “the first day of the week.” For example, we discover in Acts 20:7 that “on the first day of the week” the church in Troas “gathered together to break bread.” Based on this passage, it is apparent that the activity of “breaking bread” or observing the Lord’s Supper was associated with an assembly that took place on the first day of the week. Additionally, we read Paul’s instructions to the Corinthian congregation regarding when they should set aside their funds for “the collection for the saints.” He stated that “on the first day of every week, each of you is to put something aside and store it up” (1 Corinthians 16:2). Paul instructed the Corinthian Christians to conduct their financial collections when they assembled as a congregation so that “there will be no collecting when I come.” And Paul knew based on the practice of the early church that the congregational assemble occurred “on the first day of the every week.”
Thus, while the religious activity of the Israelites centered around the seventh day of the week, the religious activity of Christians centered around the first day of the week. What caused this shift? The first day of the week became the day of assembly for the first century church because of its association with the resurrection of Jesus. In particular, Jesus rose from the dead on “the first day of the week” (Matthew 28:1-10; Mark 16:2-8; Luke 24:1-12; John 20:1-10), and His first two post-resurrection appearances to the assembled disciples occurred on “the first day of the week” (John 20:19, 26).
It is this association between the resurrection and Sunday that would later allow the first day of the week to be identified as the “Lord’s Day” (Revelation 1:10). This expression appears only once in the text of Scripture, but there are two important observations to be made about it. First, it was frequently used in post-apostolic literature. For example, an uninspired, early Christian text called the Didache or Teaching of the Twelve Apostles, which is traditionally dated toward the end of the first century or beginning of the second century, says in 14:1 “And on the Lord's own day gather yourselves together and break bread and give thanks.” Based on this uninspired text we learn that the post-apostolic church associated the Lord’s Day with the day the church assembled to “break bread,” which according to Acts 20:7 was the first day of the week. Additionally, Ignatius of Antioch, an early Christian leader who lived from approximately 35-108 A.D., wrote, “If then those who had walked in ancient practices attained unto newness of hope, no longer observing sabbaths but fashioning their lives after the Lord's day, on which our life also arose through Him and through His death” (Magnesians 9:1). Ignatius’ letter not only indicated that the Sabbath Day was no longer a required observance within Christianity but it also acknowledged that there was a distinction between the Sabbath Day and the Lord’s Day. Based on his observation, we learn that the second century church did not associate the Lord’s Day with the Sabbath Day. Author, Wayne Jackson, summarized the importance of these extra-biblical references to the “Lord’s Day” when he wrote, “Though these writings were not inspired, they nonetheless indicate the practice of the early Christians in that era following the deaths of the Lord’s apostles.” Thus, “It is clear from the body of early historical evidence that Sunday was the day set aside by the followers of Jesus to celebrate the Lord’s resurrection.”
Additionally, it should be noted that the “Lord’s Day” terminology is reminiscent of other expressions that appear in Scripture, such as “the Lord’s cup” (1 Corinthians 10:21), “the Lord’s table,” (1 Corinthians 10:21) and “the Lord’s Supper” (1 Corinthians 11:20). All of these possessive expressions indicate a special relationship between Christ and these particular items which are affiliated with our worship assembly. Thus, John’s use of the “Lord’s Day” appears to be patterned after the same expression of ownership that Paul used in reference to the memorial that Christ instituted during His last Passover meal. And the possessive nature of this phrase indicates that the first day of the week belongs to the Lord because it was the day of His resurrection. In fact, Justin Martyr, an early Christian apologist who lived from approximately 100-165 AD, explained this connection saying (Apology 1.LXVII.3), “But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead.”
Thus, we assemble on Sunday because the first century church demonstrated a pattern of meeting on the first day of the week since it was the day that our Lord rose from the dead. But it should also be noted that we assemble more than once a week because although the New Testament presents a pattern of congregational assembly on the first day of the week, it also presents an expectation of frequent interaction between brothers and sisters in Christ. In fact, there is evidence of Christians gathering on a daily basis at least for a period of time (Acts 2:46; 5:42). The reason for assembling with frequency is identified by the author of Hebrews who told his readers to “consider how to stimulate one another to love and good deeds” and indicated that the way to accomplish this is by “not forsaking our own assembling together, as is the habit of some.” He then instructed his readers to encourage one another “all the more as you see the day drawing near” (Hebrews 10:24-25). Notice that forsaking the assembling of the church was viewed negatively not because one would miss out on worshipping God but because one would miss out on encouraging others. In fact, the author of Hebrews goes so far as to indicate that encouragement should be a daily exercise “so that none of you may be hardened by sin's deceitfulness” (Hebrews 3:13).
 Jackson, Wayne. "The Lord's Day." ChristianCourier.com. Access date: April 11, 2017.