The entire Sermon on the Mount clearly shows Jesus did "magnify the law, and make it honourable" (Isaiah 42:21). He brought it up to the spiritual level which God intended from the beginning. He "filled it full." And now, with the help of the Holy Spirit, man is capable of obeying it in its spiritual intent (Hebrews 8:10, Galatians 5:24). The New Covenant indeed magnified the Old! The New Covenant is not a "separate package" from the Old. The New Covenant is the expansion of the Old Covenant.
Paul did not merely attend the festivals. Acts 18:21 says, referring to a statement made by Paul, ". . . I must by all means keep this feast that cometh in Jerusalem. . . ." "Keep" is the Greek poiesai, from poieo, meaning "to keep, celebrate" (Analytical Greek Lexicon, by Harper, p. 332). In Acts 20:16, Paul states, ". . . he hasted, if it were possible for him, to be at Jerusalem the day of Pentecost." "To be" is the Greek genesthai, from ginomai. The Analytical Greek Lexicon by Harper (p. 79) says that the meaning of the word with respect to festivals is "to be kept, celebrated, solemnized as festivals." It would have been purposeless for Paul to have attended the festivals if he did not keep them. And if there were no need for Christians to keep the Holy Days, Paul certainly set an improper example—one that would have most surely led to confusion among the disciples. Those who say Paul did not keep the feasts find it difficult to explain what Paul was doing attending the feasts—for he did not both keep and not keep them.
The fact that Christ and Paul observed the Holy Days is ample proof they were not abrogated by the New Covenant; they are an essential part of the New Covenant. Both Christ and Paul set an example for us to follow (I Peter 2:21, I John 2:6, I Corinthians 11:1, Philippians 4:9). Those who follow Christ will keep the Holy Days.