Blainerb: "Who are the
Sabbath-keepers?" is from the following website:
http://www.religioustolerance.org/sabbath.htm, which has a good discussion on the topic.
It appears to be an assortment of
far-side religious groups, definitely not mainstream
Christianity. You may add to this list a few groups that
collectively might be referred to as Christian Judaism--they keep the
old lunar calendar, feasts, etc., even advocate animal sacrifices,
but still believe Jesus Christ was Messiah.
Mormons honor Sunday as the day of the
Lord, i.e., the day of His resurrection, and consider it the
Sabbath, but respect the Sabbaths of others, especially if
proselyting. In other words, it is not a big issue--as Kevin points
out, worship of Jesus Christ should be the central doctrinal
issue--not which day we choose to worship Him.