The authority on this is the word of God:
Lev 23:5 In the fourteenth day of the first month at even is the LORD's passover.
God SAYS that the LORD's passover is Nisan 14. Not every day. But every year on Nisan 14.
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:
God SAYS that Christ IS the passover, which makes perfect sense since death "passes over" Christian who have the Lord's spirit.
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.
The word of God and God's apostle, Paul, says LET US KEEP THE FEAST. Not "let us keep the feast". The feast that he is talking about was designated BY GOD to occur on Nisan 14, every year.
I asked for your authority that Christians kept the Passover well into the third century. Your answer is:
The authority on this is the word of God:
The word of God was completed in the first century. How would that support the keeping of the Passover in the third century. You also obviously misunderstand Paul's teaching to the first century Coninthians.