Hi there!

I hope I am not offending you by posting here. I respect your right to your opinions and I hope you will see the following simply as a look through Scripture. If I offend, please forgive me. that is not my intention.
So, you were wondering where in the NT we even see these festivals after Jesus rose from the dead. Well...
Well in
Acts 2 we see the followers of Jesus meeting in the Temple to celebrate the feast of Pentecost (which is the Biblical Holy Day
Shavuot as appointed in in Leviticus 23:15-22, Exodus 34:22; Numbers 28:26-31; Deuteronomy 16:9, 10)
So that's the first biblical feast observed after Jesus ascended to Heaven.
Acts 27:9 mentions Yom Kippur/Day of Atonement which is a holy day of fasting (actually, it's the only fast appointed by God and was always observed by faithful Jews)
That footnote points to this note:
And this proves that it was Yom Kippur because Adonai told His people:
And we see Paul in
Acts 18
There are certain feasts / holy days that require that if you can, you observe in Jerusalem. Those feasts are called the pilgrimage festivals and they are:
Passover
Pentecost
Tabernacles
So Paul was observing one of these festivals
.
Hope that helped answer your question!
Be blessed, everyone.