What is the Catholic interpretation of Romans 14:5? Romans 14:5
  One person considers one day more sacred than another; another considers every day alike. Each of them should be fully convinced in their own mind.


All kinds of blessings are in the Catholic feast days and sabbath, but here, Paul is talking about the different days that each person holds sacred.

Why should one keep a day holy if it's useless? You don't keep shabbat just to rest, there are blessings, the feast brings spiritual blessing, so if the feasts or days can't give you that anymore, why keep them?

What is the Catholic interpretation of this verse?
According to the Catholic Encyclopedia The sabbath in the New Testament:


  St. Paul enumerates the Sabbath among the Jewish observances which are
  not obligatory on Christians (Colossians 2:16; Galatians 4:9-10;
  Romans 14:5).  The gentile converts held their religious meetings on Sunday
  (Acts 20:7; 1 Corinthians 16:2) and with the disappearance of the Jewish
  Christian churches this day was exclusively observed as the Lord's Day.


Catholics interpret Romans 14:5 as meaning that the Bible supports the Roman Church's decision to move God's holy day from the Sabbath to Sunday.

For denominations that don't follow Rome's teachings, the interpretation is quite different though.  The entire chapter of Romans 14 talks about one common theme:  new Gentile converts and how they should be handled.

New converts are coming from a completely different view of the world.  Many of them are Vegetarians, Ascetics, or other sects that believe that physical deprivation improves the soul.  It will take some time for them to adjust to the new reality of Christianity, and the current Christians should be careful not to push the process too quickly.

For instance, Vegetarians believe that eating meat is a sin.  Christians know that it isn't, but if they try to force meat onto someone that still feels as if it is a sin, they risk causing that convert to leave them.  Paul points out that it is not a sin to refrain from eating meat, so there is no point in pushing this aspect of Christianity.  Instead, one should stick to the more fundamental beliefs of Christianity, and the converts will eventually accept less important aspects of their new life, such as eating meat without feeling guilty.


  Let us therefore follow after the things which make for peace, and
  things wherewith one may edify another.  For meat destroy not the work
  of God. All things indeed are pure; but it is evil for that man who
  eateth with offence. -- Romans 14:19-20


Arguing a Vegetarian into eating meat, even though he (incorrectly) feels that it is wrong, might cause him to reject his new relationship with God.

Similarly some converts might have no concept of a special day of the week, while other might think that the day the Romans dedicate to Apollo the Sun God should be treated specially.  Eventually the new converts will understand the truth, but forcing them to behave in a way they still feel is wrong is going to do far more harm than good.  It is far better to concentrate on teaching the doctrines of salvation first.