Do you have any examples that you could sight for me in the NT?
Even if there were no other references the fact that Jesus kept them should be sufficient for Christians....but nonetheless here is other scripture that proves conclusively that Christians observed God's holy days:
1Co 5:8 Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened [bread] of sincerity and truth.
Here we have a specific command from Paul to Christians in Corinith to observe the days of unleavened bread. In actuality the purpose of God including this in scripture isn't because they weren't already observing the holy day, it was to show Christians the spiritual intent.
Act 20:6 And we sailed away from Philippi after the days of unleavened bread, and came unto them to Troas in five days; where we abode seven days.
Another reference to the days of unleavened bread. Note that this occurred over 20 years after the death of Christ. Note also that Acts was written by a gentile 40 or 50 years after the death of Christ and God inspired him to include this reference.
Act 2:1 And when the day of Pentecost was fully come, they were all with one accord in one place.
The new testament church began on a holy day, Pentecost.
Act 20:16 For Paul had determined to sail by Ephesus, because he would not spend the time in Asia: for he hasted, if it were possible for him, to be at Jerusalem the day of Pentecost.
Paul wanted to be at Jerusalem for Pentecost 20 years later.
Act 18:21 But bade them farewell, saying, I must by all means keep this feast that cometh in Jerusalem: but I will return again unto you, if God will. And he sailed from Ephesus.
Probably the feast of tabernacles. Paul tells his fellow Christians that he wants to be in Jerusalem to keep the feast of tabernacles.
Act 27:9 Now when much time was spent, and when sailing was now dangerous, because the fast was now already past, Paul admonished [them],
Most scholars agree that this is a reference to the day of atonement. What's significant about it is that a gentile, Luke, writing this book under the inspiration of the holy spirit, casually includes a reference to a holy day to mark time 40 or 50 years after the death of Christ.