Jesus celebrated Passover. He is as Christian as it gets.
Cite the scripture which commands, or even condones, Christians celebrating any "holy day" whatsoever.
And no, scriptural affirmation of eating, drinking and having non-religious parties doesn't count. That's not the issue. The issue is whether elevating a festival to the status of "required by Christianity" or even just "defined by Christianity" is scripturally sanctioned.
Jesus celebrated Passover. He is as Christian as it gets.
But at that last Passover Jesus transformed it forever by instituting the New Covenant sealed by the blood of the Lamb of God, not some "year old male lamb without spot or blemish".
Celebrate the New Covenant with His Eucharist.