Can the Old Covenant festivals be seen as Christian? Does it matter that, when He was among us that the Old Covenant was in force and the New Covenant didn’t begin until He arose? When He spoke and acted, it was in obedience to the Old Covenant because He was the only one who ever walked the Earth that could be 100% obedient and deserving of and by Himself. I may not be a theologian, but it seems that many get the Old and New Covenants mixed up and fail to allow for the differences.
God's holy days were created BEFORE the old covenant was struck. They are part of the old covenant (and new) but they exist outside of any covenant.
Besides, if you believe in the concept of Christian liberty why is it "bad" for me to observe holy days created by Jesus Christ that are IN the bible...yet it's not "bad" for you to observe days that were invented by man and are NOT in the bible?
Are you aware there was more than one "Old" Covenant? Which one do you think was changed [Hebrews 7:11-12; 10:4]. Are you certain it was the Covenant that dealt with the Feast Days and Sabbaths? If you want to pursue that line of thinking you could be in for a surprise.