"And Samuel spake unto all the house of Israel, saying, If ye do return unto the Lord with all your hearts, then put away the strange gods and Ashtaroth from among you, and prepare your hearts unto the Lord, and serve him only: and he will deliver you out of the hand of the Philistines." ~ 1 Samuel 7:3
"So kill the passover, and sanctify yourselves, and prepare your brethren, that they may do according to the word of the Lord by the hand of Moses." ~ 2 Chronicles 35:6
"Why does the Lord tell us to remember the Sabbath day? The Larger Catechism (Q. 121) answers that this is “partly, because we are very ready to forget it.” Knowing our weakness, the Lord tells us, as part of his moral law, to make special efforts to keep the day for rest, worship, and works of necessity and mercy , separated unto him. (Holy means “separated unto God.”) It is sobering to realize that failure to keep the Sabbath day separated unto the Lord is on a par with idolatry, blasphemy, murder, adultery, stealing, and lying. That alone should be a powerful rebuke to us when we regard the Sabbath day as our day rather than the Lord’s, and when we seek our own pleasure rather than his (see Isa. 58: 13). If we think of the Lord’s Day for anything beyond what God has ordained it to be, we are guilty of a serious form of idolatry of self. As a major part of your preparing for Lord’s Day worship , plan ahead— that is, “remember”— to keep that whole day separate for God’s purposes for it." ~William Shishko, Helps for Worship (Kindle Locations 171-172). Committee on Christian Education.
So why did I separate this sentence of the paragraph when all of it goes together? Because setting aside a time to prepare for the Lord's Day is unfamiliar to some of us. We sometimes just go into the church building, persevere through the service, and hope that we will get something edifying from it. But what does it mean to come before God on the Lord's Day? It means coming before the Sovereign Lord of the universe; this makes the Lord's Day not just special but venerable. Therefore as children of the King, should we not ready ourselves for His appearance? Just as musicians practice their instruments before their performance in public, we ought to "practice" our hearts before we come into the presence of our gracious and awesome God the Father, Son, and Holy Spirit.