Moses 3

Much of the world seems to celebrate a 7-day week. It doesn't take much to realize that he 7-day week comes from the periods of the creation. The Lord said that -

"...on the seventh day I, God, ended my work, and all things which I had made; and I rested on the seventh day from all my work, and all things which I had made were finished, and I, God, saw that they were good;
3 And I, God, blessed the seventh day, and sanctified it; because that in it I had rested from all my work which I, God, had created and made." Pearl of Great Price Moses 3:2 - 3)

The emphasis is that the Lord "sanctified" one day in seven. Sanctifiy means -

1 : to set apart to a sacred purpose or to religious use : consecrate

2 : to free from sin : purify
3 a : to impart or impute sacredness, inviolability, or respect to b : to give moral or social sanction to
4 : to make productive of holiness or piety

The commandment to us is also to sanctify this day or to set it apart, consecrate, and make it holy.In modern day the Lord said -

"...that thou mayest more fully keep thyself unspotted from the world, thou shalt go to the house of prayer and offer up thy sacraments upon my holy day;
10 For verily this is a day appointed unto you to rest from your labors, and to pay thy devotions unto the Most High;
11 Nevertheless thy vows shall be offered up in righteousness on all days and at all times;
12 But remember that on this, the Lord's day, thou shalt offer thine oblations and thy sacraments unto the Most High, confessing thy sins unto thy brethren, and before the Lord." (Doctrine and Covenants Section 59:9 - 12)

And with doing that, the Lord promises

"...inasmuch as ye do these things with thanksgiving, with cheerful hearts and countenances, not with much laughter, for this is sin, but with a glad heart and a cheerful countenance—
16 Verily I say, that inasmuch as ye do this, the fulness of the earth is yours..." (Doctrine and Covenants Section 59:15 - 16)

Resting from our labors and sanctfying this day then means that we do it with thanksgiving and cheerful hearts. Great promises are given to those that do for -

"...in nothing doth man offend God, or against none is his wrath kindled, save those who confess not his hand in all things, and obey not his commandments." (Doctrine and Covenants Section 59:21)