“First century Christians, biblical Christians, observed God’s holy days that are listed in Leviticus 23.”
Even gentile converts? Evidence?
Yep. Paul told them to.
The church in Corinth was composed of probably MOSTLY gentiles.
1Co 12:2 You know that you were Gentiles, carried away to these dumb idols, however you were led.
And yet...
1Co 5:7 Therefore purge out the old leaven, that you may be a new lump, since you truly are unleavened. For indeed Christ, our Passover, was sacrificed for us.
1Co 5:8 Therefore let us keep the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.
Paul uses terms and concepts about Passover and the days of unleavened bread that they would not understand unless they were already keeping the feast. And by the way that word translated "keep the feast" means just that...observe one of God's holy days.