Why Sunday worship?

From creation onward, the people of God worshiped on the seventh day of the week. This was a "creation ordinance" that the Creator Himself established by His example, with the intent that His creatures would follow it. He worked six days and called His image-bearers to work (Gen. 2:15); He rested on the seventh day (Gen. 2:2; Ex. 20:11; 31:17) and called His image-bearers to rest. He signified this with His benediction, setting apart the seventh day as "holy" (Gen. 2:3).

Later, when the Sabbath command was reiterated, we read: "In six days the LORD made heaven and earth, and on the seventh day he rested and was refreshed" (Ex. 31:17). The word refreshed (Hebrew, naphash) is used only two other times in the Old Testament: once in reference to giving rest to animals, servants, and visitors within Israel (Ex. 23:12), and once in reference to David and his men (2 Sam. 16:14). After God worked to make everything, it was as if His rest refreshed Him. Yet God's rest and refreshment mean so much more; they have to do with His joy and satisfaction. The psalmist writes, "May the LORD rejoice in his works" (Ps. 104:31). God's rest and satisfaction was that of a King; having created the heavens and the earth to be His cosmic palace, He took His place on His throne, so to speak, on the seventh day.

After God brought His people out of Egypt and through the Red Sea, the Sabbath day took on even more significance as a covenant sign that God sanctified His people (Ex. 31:13). On that day, the saints celebrated the reality that God had created them and that their rest was rooted in His rest: "For in six days the LORD made heaven and earth, the sea, and all that is in them, and rested the seventh day" (Ex. 20:8-11). As well, the Sabbath signified that God had redeemed His people (Deut. 5:12-15). Finally, the annual Day of Atonement fell on a Sabbath (Lev. 16:30-31), so the Sabbath also celebrated God's forgiveness of His people.

Under the old covenant with Israel (Ex. 19; Heb. 8:6, 7, 13), the Sabbath day was extremely strict. Not only was no work to be done by the Israelites and their children, they also were to give rest to all in their households—servants, livestock, even sojourners (Ex. 20:10). God even gave regulatory laws over what could and could not be done. For example, if one even went out to gather sticks on the Sabbath in order to kindle a fire (Num. 15:32-36; Ex. 35:1-3), he was to be put to death (Ex. 31:14-15; 35:2). All this strictness was a part of the tutelage of the law, which was meant to lead Israel by the hand to Jesus Christ (Gal. 3:24), who is the final sacrifice ending the old covenant (Heb. 7:11-12, 18-19; 8:7, 13).

When Jesus rose from the dead on the first day of the week, things changed. Christ, the second Adam, "finished" (John 19:30) the work that the first Adam failed to do (Rom. 5:12-19). Because of that pivotal event, the church determined that for Christians under the new covenant, the day of worship and celebration of the Lord's grace in Jesus Christ was to be the first day of the week, Sunday: "From the beginning of the world to the resurrection of Christ, [the Sabbath] was the last day of the week; and, from the resurrection of Christ, was changed into the first day of the week, which, in Scripture, is called the Lord's day, and is to be continued to the end of the world, as the Christian Sabbath" (WCF, 21:7). 

On this day, we are reminded of and participate in the glorious reality that we have already entered God's rest (Matt. 11:28; Heb. 4:10) and that we await the experience of the fullness of this rest in eternity in the new heavens and new earth (Rev. 21-22). We now assemble corporately for worship and enjoy a foretaste of our eternal rest, then go out into the kingdom of this world to work for six days. So why do we worship on Sunday and not Saturday?

The first day of the week was the day on which our Lord rose from the dead (John 20:1; cf. Ps. 118:24).
The first day of the week is called "the Lord's day" (Rev. 1:10; cf. 1 Cor. 16:2).
The first day was the day on which the Holy Spirit was poured out on the church (Acts 2:1-36).

Just as on the first day of creation God made light and separated it from the darkness, we gather on the first day of the week to celebrate the light of the gospel in Jesus Christ, who has separated us from the world of the darkness of sin (John 1:5, 9; 3:19; 8:12; 2 Cor. 4:1-6).

From creation until Christ, the people of God worked six days and then rested on the seventh day. This was a picture of their looking forward to eternal rest; the seventh day of creation was not structured with an "evening and morning" as the previous six days (Gen. 2:1-3), which signified that the seventh day had no end and was thus a foretaste of eternity itself. On the other hand, from the work of Christ until the consummation, the people of God rest on the first day and work the next six, looking back on the finished work of Christ. Yet we too look forward to the full consummation of this rest.

This excerpt is taken from Welcome to a Reformed Church by Daniel Hyde.