God says His Sabbath is a divine appointment that He commands His people to keep through their gathering before Him with other believers.
In Leviticus:23:2-3 God reveals an important aspect of the weekly Sabbath day and His other annual festivals: "Speak to the Israelites and say to them: 'These are my appointed feasts, the appointed feasts of the Lord, which you are to proclaim as sacred assemblies . . . the seventh day is a Sabbath of rest, a day of sacred assembly'" (New International Version, emphasis added throughout).
God plainly says these are His feasts, His "sacred assemblies." The Hebrew word mo`ed, the plural form here translated "appointed feasts," means "appointed time" or "meeting" (Lawrence Richards, Expository Dictionary of Biblical Words, 1985, "Feast/Festival"). "Sacred assemblies" and "sacred assembly" here are translated from miqra, which denotes a divinely summoned gathering.
In other words, God says His Sabbath is a divine appointment that He commands His people to keep through their gathering before Him with other believers (compare Hebrews:10:24-25).
Notice that God is the one who sets the appointment, not us. He is the one who determines the time—that time being His weekly seventh-day Sabbath and His annual festivals. Weekly Sunday worship does not fulfill God's command.
That raises an interesting question: If we don't come before Him at the time He commands, either by not coming at all or coming on a different day, are we really keeping an appointment with Him?
If you make arrangements to meet with someone next Wednesday but he decides to show up on Thursday instead, would you think he had kept the appointment? Of course not. So why should we think God would find it acceptable if we decide to assemble on a day different from the one He commands?
The Sabbath is God's day, not ours. It is a time He wants to meet with you, a time for reading His Word, for prayer, for fellowship with other believers, for your family—but, most of all, a time for His presence with you, especially as you are taught from His Word at His commanded assembly.