Exodus 20

The Lord spake the words -

"Remember the sabbath day, to keep it holy.

9 Six days shalt thou labour, and do all thy work:
10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates:
11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it."  (Old Testament | Exodus 20:8 - 11)

What did He mean by keeping it holy?  In modern day, the Lord instructed -

"Thou shalt offer a sacrifice unto the Lord thy God in righteousness, even that of a broken heart and a contrite spirit.
9 And 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.
13 And on this day thou shalt do none other thing, only let thy food be prepared with singleness of heart that thy fasting may be perfect, or, in other words, that thy joy may be full.
14 Verily, this is fasting and prayer, or in other words, rejoicing and prayer."  (Doctrine and Covenants | Section 59:8 - 14)

I believe the word "fasting" in this revelation is more in reference to fasting or avoiding those things that we do six days of the week that are not considered spiritual in nature.

The promise of obedience is, or the if...then is -

"....that inasmuch as ye do this, the fulness of the earth is yours, the beasts of the field and the fowls of the air, and that which climbeth upon the trees and walketh upon the earth;
17 Yea, and the herb, and the good things which come of the earth, whether for food or for raiment, or for houses, or for barns, or for orchards, or for gardens, or for vineyards;
18 Yea, all things which come of the earth..."  (Doctrine and Covenants | Section 59:16 - 18)

Isaiah also explained the blessings of keeping the Sabbath day holy -

"If thou turn away thy foot from the sabbath, from doing thy pleasure on my holy day; and call the sabbath a delight, the holy of the LORD, honourable; and shalt honour him, not doing thine own ways, nor finding thine own pleasure, nor speaking thine own words:

14 Then shalt thou delight thyself in the LORD; and I will cause thee to ride upon the high places of the earth, and feed thee with the heritage of Jacob thy father: for the mouth of the LORD hath spoken it."  (Old Testament | Isaiah 58:13 - 14)

In my mind, "ride upon the high places" would suggest having an eternal perspective and the heritage of Jacob would be the promises of exaltation and becoming a great nation.