LBCF, Chapter 22, Para. 7:
“[God] hath particularly appointed one day in seven for a sabbath to be kept holy unto him, which from the beginning of the world to the resurrection of Christ was the last day of the week, and from the resurrection of Christ was changed into the first day of the week, which is called the Lord’s day: and is to be continued to the end of the world as the Christian Sabbath, ….”
“Nevertheless, it is lawful and necessary, upon special emergent occasions, to separate a day or days for publick fasting or thanksgiving, as the several eminent and extraordinary dispensations of God’s providence shall administer cause and opportunity to his people.”
~Westminster Directory for Public Worship
Westminster Confession of Faith, Chap. 21:
7. As it is of the law of nature that, in general, a due proportion of time be set apart for the worship of God; so, in his Word, by a positive, moral, and perpetual commandment, binding all men in all ages, he hath particularly appointed one day in seven for a Sabbath, to be kept holy unto him:a which, from the beginning of the world to the resurrection of Christ, was the last day of the week; and, from the resurrection of Christ, was changed into the first day of the week,b which in Scripture is called the Lord’s day,c and is to be continued to the end of the world, as the Christian Sabbath.d
a. Exod 20:8, 10-11; Isa 56:2, 4, 6-7. • b. Gen 2:2-3; Acts 20:7; 1 Cor 16:1-2. • c. Rev 1:10. • d. Exod 20:8, 10 with Mat 5:17-18.
8. This Sabbath is then kept holy unto the Lord, when men, after a due preparing of their hearts, and ordering of their common affairs beforehand, do not only observe an holy rest all the day from their own works, words, and thoughts, about their worldly employments and recreations;a but also are taken up the whole time in the public and private exercises of his worship, and in the duties of necessity and mercy.b
a. Exod 20:8; 16:23, 25-26, 29-30; 31:15-17; Isa 58:13; Neh 13:15-22. • b. Isa 58:13; Mat 12:1-13.