I think the principle in Romans 14:5-6 is apropos here:

"One person considers one day to be above another day. Someone else considers every day to be the same. Each one must be fully convinced in his own mind. Any followers who count one day more important than another day do it to honor their Lord."

Non-Jewish Christians were never commanded to observe Passover, or Christmas. 1 Corinthians 5:7 states that Christ is our Passover. However, given all that Passover symbolizes concerning Jesus, I don't see how it would be harmful to celebrate it, as long as you keep Christ at the center of it. It would be the same with Christmas, in my mind. Just remember that Jesus is the reason for the season.

Some people, like Jehovah's Witnesses, don't celebrate birthdays. I've heard it said that only pagan kings in the Bible celebrated their birthday, like pharaoh or King Herod. However, when Jesus was born, His Father threw Him the biggest birthday party in the universe, complete with The Star of Bethlehem, choirs of angels, and visitation by shepherds and eventually the Magi. So if you look at it this way, God thought Christmas was a big deal!

So whether you observe Passover or Christmas, or not, do it for the Lord's sake, and you will be all right, and no one should judge you for it (Romans 14:5-12).