I need to write happy Christmas for friends. How do I do it?

You've left it a bit late for this year.

I'm not going to presume to know more about your relationship than you do. The obvious answer is to write or type the words happy Christmas in a letter, card or email, and then send it through the post, electronically or deliver it by hand.

Next year.

This year, along with whatever else you have to tell them in your message, you include the words ‘ I hope you (and your family) had a very happy christmas.'

Easy.