I personally feel, in God’s eyes, all days are the same.

My understanding is that Constantine in order to smooth the conversion of The Roman Empire to Christianity “Christianized” currently practiced festival days. Considering they are still practiced today, I would say it was effective.

Today they may be effective for the same reason. Everywhere there’s a babe in a manger… Music, cards. Yes it’s mostly a superficial nod to Christianity, but it pull people’s heart to spiritual things. It gives those of us who live for this God turned Child in a manger turned Savior of the world year-round an opportunity to tell people about Him.

The phrase fully convinced in the Romans passage is important I think. Are you fully convinced in your own mind (given the fact that to practice holidays and food restrictions seem not to be an area considered sin) to do a thing or not?

1 Corinthians 8 and 10 speak on matters of conscience.

I personally believe revisiting the OT feasts would add a richness and a connection to the vastness of God’s plan of salvation. I have attended Messianic festivals in Monroe and Synagogue with my son and daughter-in-law when I was in Washington years ago and I love the connection between the Old Testament and Christ.

Though I am free and belong to no one, I have made myself a slave to everyone, to win as many as possible. To the Jews I became like a Jew, to win the Jews. To those under the law I became like one under the law (though I myself am not under the law), so as to win those under the law. To those not having the law I became like one not having the law (though I am not free from God’s law but am under Christ’s law), so as to win those not having the law. To the weak I became weak, to win the weak. I have become all things to all people so that by all possible means I might save some. I do all this for the sake of the gospel, that I may share in its blessings.
1 Corinthians 9:19‭-‬23 NIV