Which Christian Denomination or Sect don't celebrate Christmas or Easter?

The 2 that I know of are the Jehovahs Witnesses and the Messianic Jews, who think of themselves as Jewish.

Historically, the Puritans did not, either.

There may be others among the thousands of Christian sects, but those are the ones I know about.

It wold be a contradiction for any Christian denomination not to celebrate Christmas.

Any Christian is hailed thus because they follow the teaching of CHRIST.

Now the essence of the Christian religion is Easter. If Jesus had not risen than Christianity would have no reason to exists. Thus their cannot be anybody who believe in Jesus if they do not understand that Jesus had risen for us.

Mid Acts Dispensationalism - We're not a denomination; nor do we worry about holidays of any kind due to what Paul says in Colossians. If you do it, you do it. If you don't, you don't. The world keeps on going, and our job is to see souls saved, and saints edified, not do Christmas and Easter.