It's in Hebrews. Hebrews chapter 8, 9 and 10 clearly present that the Levitical priesthood has been replaced by a new, better priesthood in Christ. Chapters 9 and 10 especially make clear that animal sacrifices and burnt offerings are not needed. HOWEVER, nowhere does it say that God's Holy Days are not to be observed, or are of no effect.
Obviously the writer of Hebrews wasn't afraid to buck the system of Judaism. He made a major change by outlining why sacrifices and the priesthood were not needed. Yet he did not make a case that God's Holy Days were no longer to be observed. It is certain that in biblical times Christians celebrated the Lord's feast days. They certainly didn't celebrate Easter, Christmas or any other holiday that we associate with Christianity today.
Oops. Sorry. Fault.
If you read very carefully, or actually not so carefully, you will see that Hebrews is speaking of a change from one system to another; from the Levitical Order of the sacrificial system to the Melchizedekian Order of Jesus Christ. "As He also says in another place: 'You are a priest forever According to the order of Melchizedek'" (Heb. 5:6). It was not a change of the Levitical Order. It was an abolishment of the Levitical Order.
The "holy days" were part and parcel of the Levitical system. Melchizadek never kept the festival days of Leviticus.
"But now He has obtained a more excellent ministry, inasmuch as He is also Mediator of a better covenant, which was established on better promises." (Heb. 8:6)
The imperfect Levitical system was completely replaced by the perfect one after the Order of Melchizedek.
Try again. Where is there an divinely authorized change to the Levitical system so that bloody festivals become unbloody. (I'll give you a hint, there is only one, and it is directly referenced in the establishment of the new covenant.)