Note: Texts are quoted from the New American Standard Bible (NASB) unless otherwise noted.
One man regards one day above another, another regards every day alike. Let each man be fully convinced in his own mind (Romans 14:5).
This text is sometimes used to support the view that the Sabbath is a matter of indifference for Christians. Some take that a step further and say that for Christians all days are holy; thus there is no need to "keep" one day as better than the others.
These interpretations have special appeal for those who oppose the observance of the seventh-day Sabbath. What does this verse really mean?
Let's take a look at Paul's introduction to this portion of his letter to the Romans:
Now accept the one who is weak in faith, but not for the purpose of passing judgment on his opinions (Romans 14:1).
Please note that the apostle is writing about "opinions." The King James Version has "doubtful disputations" instead of "opinions." The New International Version has "disputable matters."
These various terms give us a key to understanding verse 5. Paul is talking about "the anxious internal debates of conscience"  that lead to differences in areas of personal preference. He is advising the Roman Christians not to let their views on such things divide them. He starts with personal dietary preferences:
The one who eats is not to regard with contempt the one who does not eat, and the one who does not eat is not to judge the one who eats, for God has accepted him. (Romans 14:3).
Some of his readers are vegetarians, others are not. The two groups should not be critical of each other. They all serve the same Master. In the following verse Paul asks, rhetorically, "Who are you to judge the servant of another?" This is the immediate context of our target verse.
In verse 5 Paul shifts the focus away from diet to the observance of special days. "One person regards one day above another, another regards every day alike." Several versions have "esteems/esteemeth" instead of "regards."
"One day above another" is literally "a day above a day." There are several different views on what the "one day" is. Some scholars think this may be a reference to the unlucky days of the pagans or to a syncretistic  religious calendar that promoted "the peculiar powers of different days of the year."  However, the first part of verse 6 shows that Paul is referring to days that fall within the Judeo-Christian context:
He who observes the day, observes it for the Lord.
» Some Bible scholars see the annual festivals of the Jews here. Others think that Paul is talking about days designated for fasting. "The special days of the week referred to in our passage were probably fast days. This suggestion is based on the context itself, in which abstinence is the predominant feature."  And of course there are those who agree that the reference is to the weekly Sabbath. 
"Every day alike." The word "alike" has been added by translators. It does not appear in the original Greek. Thus the comparison Paul sets up reads, more literally, like this: "One man judges (esteems) a day above a day, another judges (esteems) every day." With insight provided by verse six, we could put it this way: "One man observes one day (for the Lord), while another man observes every day (for the Lord)."
Anglican theologian Handley C. G. Moule seems to agree with that. He comments, "It describes the thought of the man who, less anxious than his neighbor about stated ‘holy days,’ still aims not to ‘level down’ but to ‘level up’ his use of time; to count every day ‘holy,’ equally dedicated to the will and work of God." 
Some people assume that verse 5 is talking about the Sabbath and come up with something like this: "One man observes the Sabbath once a week (on the seventh day, of course) while another man observes every day as the Sabbath." But as the verse continues it becomes clear that Paul is not talking about the Sabbath.
"Let each man be fully convinced in his own mind." These words remind us that Paul is giving practical advice at the level of personal opinion—"disputable matters." These are not issues that have been settled by divine revelation through angels, prophets, or apostles. These are things that people can decide for themselves—matters of personal preference or conviction.
This automatically eliminates from consideration all points of doctrine that are indisputable because they are based on divine injunction or on other authoritative teachings from the Word of God. So observance of the Sabbath cannot be at issue in Romans 14. After all, the Sabbath "was enshrined among the eternal sanctities of the Decalogue, uttered…amidst the terrors of Sinai." 
The Wesleyan Bible Commentary puts it this way: "Of course this whole discussion concerns matters on which God has not spoken clearly in His word. No such questions can be conscientiously raised concerning the fundamental moral issues that are clarified in the Decalogue, the Sermon on the Mount, or in any other plain statement of Scripture. When God has spoken there is no other legitimate side to the issue." 
On the other hand, traditional Jewish festivals are not eliminated from consideration since they are part of a figurative ceremonial system that met its fulfillment in the ministry and death of Christ. It’s only natural that a Jewish convert to Christianity—having a tender conscience—would continue to honor those holy days. According to Paul, there is nothing wrong with that as long as it is characterized by sincere worship of the Lord.
This brief look at Romans 14:5 reveals that Paul’s counsel has nothing to do with the observance or non-observance of the seventh-day Sabbath. This conclusion is supported by the fact that neither biblical nor historical records give any indication that Sabbath observance was an issue in Paul’s time.
The days under consideration may be Jewish festival days or other days for feasting or fasting, but—whatever they are—Paul leaves the question of their observance up to the individual. He does the same with dietary preferences. In such matters he teaches that Christians should mind their own business and not make mountains out of molehills. Good advice.
 Handley C. G. Moule, The Epistle of Paul to the Romans, The Expositor’s Bible, ed. W. Robertson Nicoll (New York: A. C. Armstrong and Son, 1899), 374.
 A blending of diverse beliefs, i.e., Christian and pagan.
 Herold Weiss, A Day of Gladness (Columbia: University of South Carolina, 2003), 122.
 Raoul Dederen, “On Esteeming One Day as Better Than Another – Romans 14:5, 6” in The Sabbath in Scripture and History, Kenneth A. Strand, editor (Washington: Review and Herald Publishing Association, 1982), 336.
 C. E. B. Cranfield, A Critical and Exegetical Commentary on The Epistle to the Romans, vol. ii, The International Critical Commentary, ed. J. A. Emerton and C. E. B. Cranfield (Edinburg: T. & T. Clark Limited), 705.
 Moule, Romans, 375.
 Robert Jamieson, A. R. Fausset, David Brown, Commentary Practical and Explanatory on the Whole Bible (Grand Rapids: Zondervan Publishing House, 1973), 1177.
 Wilber T. Dayton, Romans and Galatians, Wesleyan Bible Commentary Vol. 5 (Grand Rapids: William B. Eerdmans Publishing Company, 1965), 86.