Hurry DouglasKC. (raising hand in the back row) "I know. I know". let me. let me.
New testament scripture, especially the book of Hebrews, makes it clear what has been changed under the new covenant. Boiling it down, the priesthood has changed from the Levitical priesthood to once in which Christ is the high priest. Most of that is discussed in Hebrews 7. Animal sacrifices have been changed to one in which the sacrifice of Christ is sufficient..this is discussed in depth in Hebrews 8,9 and 10. Since there was change in the priesthood all laws and ordinances related to the priesthood have been altered.
BUT the holy days of the Lord were NOT just part of the Levitical priesthood. They were not given to the priesthood but instead to the children of Israel (Lev 23:2). Various scriptures show they predate the old covenant and, according to scripture, will be observed and celebrated when Jesus Christ returns.
This is a rather large topic and I've only scratched the surface but we can discuss further if you would like.