Because scripture shows throughout the new testament that the holy days of Jesus Christ were observed by gentiles Christians as well as Jews. In addition the holy days of the Lord were given before the old covenant was struck with Israel. So even though they are included as part of the old covenant, they span all covenants. The observance of the Lords' holy days are also seen in the future millennial kingdom after Christ returns. I realize you will not be able to accept that, but there are plenty of scriptures that show this to be show. I'll provide them if you wish but they're also listed in the links above I gave to caww.
As pointed out earlier the issue of circumcision is well explained and well documented in the new testament. Paul and other Christians did not obviate or do away with circumcision. They only came to the realization that it was not needed for salvation since gentiles received God's spirit without the benefit of physical circumcision. In addition, they discerned through scripture the spiritual equivalent of circumcision, which is circumcision of the heart...a tender and sensitive heart that is critical for obedience and love toward the Lord and toward others.
I think circumcision goes waaaaaay back.
Were all the gentiles observing the jewish holy days and dietary laws?
So Jesus changed it - in scripture?