Amen! The Greek word "sabbatismos" is used no where else in the Bible. It's amazing that Sabbatarians would suggest that this is the word for "keeping the weekly Sabbath" when it is never used anywhere else, in spite of the many references to Jews keeping the weekly Sabbath in the New Testament.
W. E. Vine, Greek Dictionary on "Rest" proves the Sabbatarian argument wrong:
A2. KATAPAUSIS (2663), in classical Greek, denotes a causing to cease or putting to rest; in the N.T., rest, repose; it is used (a) of God's rest, Acts 7:49; Heb. 3:11, 18; 4:1, 3 (twice), R.V. (1st part), "that rest" (the A.V., "rest," is ambiguous), 5, 11; (b) in a general statement, applicable to God and man, 4:10.
A4. SABBATISMOS (4520), a Sabbath-keeping, is used in Heb. 4:9, R.V., "a Sabbath rest," A.V. marg., "a keeping of a Sabbath" (akin to sabbatizoµ, to keep the Sabbath, used, e.g., in Ex. 16:30, not in the N.T.); here the Sabbath-keeping is the perpetual Sabbath rest to be enjoyed uninterruptedly by believers in their fellowship with the Father and the Son, in contrast to the weekly Sabbath under the Law. Because this Sabbath rest is the rest of God Himself, 4:10, its full fruition is yet future, though believers now enter into it. In whatever way they enter into Divine rest, that which they enjoy is involved in an indissoluble relation with God.