The Day of Worship

The mark of the beast is a worship issue.

And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive his mark in his forehead, or in his hand.
- Revelation 14:9 (KJV)

Worship has to do with God's law, the Ten Commandments.

If ye love me, keep my commandments.
- John 14:15 (KJV)

The day of worship is the Sabbath (Friday sunset - Saturday sunset).

And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith the LORD.
- Isaiah 66:23 (KJV)

Satan changed the day of worship to Sunday.

And he [satan] shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.
- Daniel 7:25 (KJV)

He did so with the help of the Roman Catholic Church.

Of course the Catholic Church claims that the change [from Sabbath to Sunday] was her act. And the act is the mark of her ecclesiastical power and authority in religious matters.
- Faith of Our Fathers, p. 14

The seventh-day Sabbath is the day of worship, not Sunday.