If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, the same shall drink of the wine of the wrath of God. Rev. 14:9, 10.
The seal of God in the people's foreheads at the very end has to do with the Sabbath, the day of worship in honor of the Creator. Therefore the mark of the beast must have something to do with a day of worship as well.
The mark-of-the-beast issue is far deeper than simply a day to go to church. The common thread going through all the three angels' messages is a warning against self-worship. The thing that is so tragic about this beast power is that it is an organized group of self-worship, that even thinks it can change God's laws and His times. You cannot go through those scriptures without being startled at how deeply God feels about the issues. There are many people who know that the Sabbath is the day of worship taught by Scripture, but they have not realized how God feels about it. The reason God feels this way is because He gave it in the first place in honor of Creation.
If I were the devil, and I saw a day given every week in honor of the Creator, the One who made all things, I would be so upset that I would have to do something about it. If there's anything greater than saying that God made a tree or a mountain or an ocean, it is that God made everything--"All that in them is." You can't get any bigger than that. So Satan had to do something about the day of worship in honor of that. God has sent us special information that this would happen, and when it would happen, and how God would feel about it, so that we would not be caught unawares. Jesus is revealed in this doctrine in His feelings about the day of worship, about His law, and about a group of people who go along with Satan's deceptions, and a group who stand loyal to Him to the very end. "I saw as it were a sea of glass mingled with fire: and then that had gotten the victory over the beast, and over his image, and over his mark, and over the number of his name" (Rev. 15:2). Don't you want to be in that group that has gotten the victory over self-worship, over independence, over trying to be God?