What passed away under the new covenant is specified in the new testament. Among these things are the Levitical priesthood. However, scripture does not say that God's Holy Days are passed away. On the contrary, God's feast days were actually instituted before the old covenant was ratified (Passover and Unleavened bread were revealed before Israel left Egypt) which means they fall outside of any covenant. This is evident because end time prophecies specifically say they will be observed in the future:
Zec 14:16 And it shall come to pass, [that] every one that is left of all the nations which came against Jerusalem shall even go up from year to year to worship the King, the LORD of hosts, and to keep the feast of tabernacles.
Zec 14:17 And it shall be, [that] whoso will not come up of [all] the families of the earth unto Jerusalem to worship the King, the LORD of hosts, even upon them shall be no rain.
And indeed since God takes great pains to call them "HIS" feast days it's clear that they don't belong to any covenant or even any one nation.
They were feast days that were observed only by the Jewish nation, but not by the universal Christian church,
They were cetainly observed by the early church...i.e. the church of scripture and by many Christians throughout history. You're argument is correct in that the traditional Christian church has by tradition not observed them but that is due mainly to the anti-semitism that became prevelent early in, and throughout, church history.
since they were tied to the temple which was destroyed.
The holy days were not specfically tied to a temple in Jerusalam. In fact, Israel was a nation and observed the holy days forr 500 years before there was a temple. In addition, scripture never specfies that the holy days can only be kept in Jerusalem in a temple.
"After this there was a feast of the Jews, and Jesus went up to Jerusalem." "Now the Passover, a feast of the Jews, was near." "Now the Jews' Feast of Tabernacles was at hand."
Paul never encouraged anyone to observe these days as part of an ongoing new covenant tradition.
It was the normative practice of the new testament church to observe the holy days. The new testament church was began on a holy day (Pentecost). Christ was crucified on Passover.
In addition, Paul specifically instructed the Christian churches to keep the feast of unleavened bread, albeit with a new understanding due to the new covenant:
1Co 5:7 Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
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.
Paul also kept them:
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.
And of course Christ kept the holy days..no surprise since he is God and they are God's holy days.
Interesting... OK, this is a stretch, but here goes...
The Feast of the Tabenacles was an eight-day celebration of the Dedication of the Temple of Solomon. That Temple was destroyed, and rebuilt. The feast of the dedication of the 2nd Temple is Christmas (Chislev 25th). So is it too much of a stretch to keep the feast of the Dedication of the Temple by celebrating the 25th of Chislev, when the Spirit of God came to dwell among Men?