I recently had a Jewish friend ask me about Jewish Christians (Jews for Jesus) keeping to their Jewish rituals and practices. He seemed to think there was something odd about that. Although I didn't go into detail with him, I told him that I thought it was unnecessary but that I didn't think there was anything wrong about it. If a practicing Jew converts and becomes a Christian, why shouldn't he keep to the Jewish traditions he knows and loves?
We Christians are used to a whole slew of differing customs and practices. I'm not sure how many Christian denominations there are, but each of them has its own flavor. If one compares the Amish to Charismatic Pentecostals to Roman Catholics one may begin to see how varied and colorful Christian experience can be. And why not? Being a Christian has nothing to do with which church you go to, or if you go at all. It has nothing to do with what you wear, or how you prepare your food, or the days on which you choose to celebrate God's grace. So, keeping Kosher or Passover Seder is really no less Christian than any other worshipful observance or practice.
So, does this mean that anything goes? No, of course not. Sin is still sin. Christians believe in the God of Abraham, Jesus Christ, and the Holy Spirit. Most Christians recognize the Trinity, and a few think of God as a single or dual entity. Regardless, the key Christian concepts are belief in God the Father, belief in Jesus as the Son of God, and belief that God's Spirit may be with us. Any ritual or practice that denies those beliefs, or introduces any other object of worship is pagan and sinful. But that leaves a multitude of options for Christians to enjoy.