The fourth commandment, found in Exodus 20:8-11, makes a claim for the seventh-day Sabbath, of which adherents of no other day can boast: "[T]he Lord . . . rested the seventh day. Therefore the Lord blessed the Sabbath day and hallowed it" (verse 11; see also Genesis 2:3).
Men can set apart a day, as they have done with Sunday, Christmas and Easter, and with holidays of all stripes, but doing so does not mean God accepts it. He made the Sabbath and His festivals holy (Leviticus 23:2-3). For anything to be truly holy, it must have its source in Him. Only God can hallow a person, an object, or a period of time. Everything else is either common or profane.
Much of Christianity has virtually ignored the biblical laws of holiness by declaring that, in a spirit of magnanimity, God will accept just about any kind of worship based on what is in a person's heart. This, however, shows at least ignorance of, if not rebellion against, His laws of holiness.
We can see the basis of these laws in Leviticus 19:1-3: "And the Lord spoke to Moses, saying, "Speak to all the congregation of the children of Israel, and say to them: 'You shall be holy, for I the Lord your God am holy. Every one of you shall . . . keep My Sabbaths: I am the Lord your God.'" It is God's involvement, purposes, and commands that confer holiness. He commands us to keep His Sabbaths because He is in them, thus they are holy.
The seventh-day Sabbath is the holy time God has sanctified for His people to meet with and worship Him. No man or group of men has the authority or purity to hallow any other time for this purpose.