A Sign
Seal, mark, and sign are all synonymous.
The mark is a sign that you worship the beast.
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)
The sabbath is a sign that you worship God.
Wherefore the children of Israel shall keep the sabbath, to observe the sabbath throughout their generations, for a perpetual covenant. It is a sign between me and the children of Israel for ever: for in six days the Lord made heaven and earth, and on the seventh day he rested, and was refreshed.
- Exodus 31:16-17 (KJV)
The Sabbath is clearly a set day for worship, and is part of the Ten Commandments.
Currently there are only two worship days that compete with each other.
- Sabbath
- Sunday
If you keep the Sabbath (Seal of God), it is a sign that you worship God.
If you keep the Sunday (Mark of the beast), it is a sign that you worship satan.