This is by no means an exhaustive treatment of Calvin's view on Christians celebrating holy days. With so many of Calvin's theological progeny observing holy days (other than the Lord's Day-Sabbath), I have to write something on this.
First, there is an interesting statement from the French Confession worthy of quoting on this issue of the observance of ceremonies, which I believe is connected with the observance of holy days: "We believe that the ordinances of the law came to an end at the advent of Jesus Christ; but, although the ceremonies are no more in use, yet their substance and truth remain in the person of him in whom they are fulfilled." Calvin co-wrote this confession with one of his students, and it was published in 1559. This article (23) teaches that all of the ceremonial law has been rendered obsolute because it has been fulfilled in Christ.
Christ is the end of the law, according to Paul. Tied with the ceremonial law is the observance of Jewish festival days such as Passover, Pentecost, etc. Since God had commanded that the Jews keep these days they were in effect until changed. We have apostolic testimony that these feasts no longer have currency with the people of God. Paul makes this clear in two passages of Scripture: Gal 4:9-10 and Col 2:16-17. Calvin comments on the latter passage with some clear instruction: But some one will say 'We still keep up some observance of days.' I answer, that we do not by any means observe days, as though there were any sacredness in holidays, or as though it were not lawful to labour upon them, but that respect is paid to government and order--not to days." The immediate context of the passage has to do with Jewish mystical teachers (possibly Gnostics) who implored Gentile Christians to be circumcised and observe the Law including holy days. If it was wrong for Jewish teachers to impose upon Christians the celebration of biblical (OT) holy days, it is wrong for the Church to impose on Christians the celebration of holy days that have no warrant in NT.
Calvin's teaching (and Reformed teaching) was that Christians have been freed from the annual observance of festival days. The OT days have been abrogated owing to Christ's coming in fulfillment of the types and shadows. Christ is our Prophet, Priest, and King has given us no more annual festival days to celebrate. Where are they? Christmas, Easter, etc. are the creations of man, and have no binding power on the conscience of any Christian.
As Reformed Christians (or Reforming Christians) we must examine in the light of Scripture the things we do and celebrate in the name of Christ. Christmas and Easter diminished the weekly Sabbath. I know that Easter is always on the Sabbath, but Easter is celebrated as though it is holier than any other Sabbath. This is unbiblical. Every Lord's Day is a commemoration of Christ's resurrection from the dead because that represents his finished work of redemption for all of his people. Let us delight in the Sabbath always.