It would depend upon the religious belief of the person asking the question. To the Jew, the seventh day (Saturday) is the one to be observed. It was commanded in the Law given to the Israelites that they should keep the seventh day as a holy day. That Law is still binding upon the Jews. If a person is a Seventh Day Adventist, then let him keep the seventh day also, for he believes that he is under the Law, and with that belief he certainly should keep the seventh day. To those who are members of one of the various Protestant Churches, or of the Roman Catholic Church, let them observe the first day (Sunday) as the day of rest, as they believe that it is so commanded, and surely it would not be right for them to violate their conscience by doing otherwise. 

We are to praise the Lord every day. Hebrews 4:1-3 tells us we have entered into our Lord’s rest, i.e. Sabbath. The lesson of the Sabbath is more than setting aside one specific day of the week to worship God. It is a daily worship. We can be at peace, resting by trusting and loving God. Luke 9:23: (NIV) “Whoever wants to be my disciple must deny themselves and take up their cross daily and follow me.”

To the true Christian, the consecrated child of God, let these keep every day holy unto the Lord and not merely one day in seven. And let these enjoy a rest by faith in the Lord and in His sure promises. Let no one attempt to judge another in respect to this matter of observing the sabbath or holy days. 

No one has the right to judge his fellowman in this regard; but let every man be fully persuaded in his own mind. Romans 14:4-5 (NKJV), “Who are you to judge another’s servant? To his own master he stands or falls. Indeed, he will be made to stand, for God is able to make him stand. One person esteems one day above another; another esteems every day alike. Let each be fully convinced in his own mind.” 

If any one presumes to judge another in these matters, he is violating a direct command of the Lord as expressed by one of His inspired Apostles (Colossians 2:16-17 (NKJV), “So let no one judge you in food or in drink, or regarding a festival or a new moon or sabbaths, 17 which are a shadow of things to come, but the substance is of Christ.” If any one should say that we are not saved and that we are lost because we do not observe some special day, ask him gently but firmly–“Who made thee a judge?“

Additional Resources:
Christian Questions Podcast Episode #1127: “What Does the Sabbath Look Like for Christians?”
Discovering what the Sabbath really means for Christians
Preview Video 
CQ Rewind Show Notes

Sermon: (Audio only) “The Rest of the Story,” Allen Springer 
(Video) Video Version