From Romans and Galatians in particular, it is clear that the Old Law is not binding on anyone because it only pointed forward to Christ.
But before faith came, we were kept under the law, shut up unto the faith which should afterwards be revealed. Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. But after that faith is come, we are no longer under a schoolmaster. For ye are all the children of God by faith in Christ Jesus. (Galatians 3:23-26)
It is the opinion of St. Thomas (which should not be dismissed lightly), that observing the ceremonial laws is a mortal sin. At the very least, I think it is a slap in the face to God to live as if Christ never came. By observing the old laws, you are outwardly professing faith in a Christ yet to come while Jesus Christ already came and gave up his life for you.
On the contrary, The Apostle says (Galatians 5:2): “If you be circumcised, Christ shall profit you nothing.” But nothing save mortal sin hinders us from receiving Christ’s fruit. Therefore since Christ’s Passion it is a mortal sin to be circumcised, or to observe the other legal ceremonies.
I answer that, All ceremonies are professions of faith, in which the interior worship of God consists. Now man can make profession of his inward faith, by deeds as well as by words: and in either profession, if he make a false declaration, he sins mortally. Now, though our faith in Christ is the same as that of the fathers of old; yet, since they came before Christ, whereas we come after Him, the same faith is expressed in different words, by us and by them. For by them was it said: “Behold a virgin shall conceive and bear a son,” where the verbs are in the future tense: whereas we express the same by means of verbs in the past tense, and say that she “conceived and bore.” In like manner the ceremonies of the Old Law betokened Christ as having yet to be born and to suffer: whereas our sacraments signify Him as already born and having suffered. Consequently, just as it would be a mortal sin now for anyone, in making a profession of faith, to say that Christ is yet to be born, which the fathers of old said devoutly and truthfully; so too it would be a mortal sin now to observe those ceremonies which the fathers of old fulfilled with devotion and fidelity. Such is the teaching Augustine (Contra Faust. xix, 16), who says: “It is no longer promised that He shall be born, shall suffer and rise again, truths of which their sacraments were a kind of image: but it is declared that He is already born, has suffered and risen again; of which our sacraments, in which Christians share, are the actual representation.”
newadvent.org/summa/2103.htm#article4