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.
You want a command. A list of requirements that you MUST do. I see. That would be a formula for going to heaven. Which is works. Works don't get you to heaven, faith does. But you prove your faith by your works.
If you wish to be grafted into the Vine, you must conform. Otherwise go do your own thing. Christ is the example. Follow His Example as He did in real life. Jesus said "If ye love Me, keep my commandments". So, if you want a command, there it is.
The Last Supper was a Passover Sedar. “Why is this night different than the others?” Big time.
Here is the same writing from P. Schaff’s translation
http://www.ccel.org/ccel/schaff/anf01.ix.viii.iii.html