I think just saying Merry Christmas doesn't get the point across. So I say
"Merry Christmas? Christmas is a HUMBUG.
A pox on all your Christmases, and your little dog's too.

May your tree catch fire and your pudding not.
May all your presents be little ones.
May Carol singers pen you in your home for hours when you have just discovered you need your last fifty pence for the electricity meter.
May you discover your turkey does not fit in your oven, twenty minutes before all your relatives are due to come round to your house for dinner.
May Santa fall off your roof top and slip a disc, cancelling Christmas for everyone.
May you spend boxing day talking to the vegetables on your plate because they're more interesting than your relatives.
May your Bread Sauce and your mashed potato be indistinguishable from each other in lumps, and taste and texture.
May all your brussel sprouts be hard and cold, and The Sound of Music be constantly on the TV. (Nothing new there then).
So Merry Christmas to you, and to all a Goodnight."

Velvet.