The pair are expected to be spending the day with the Middleton family at their home in Berkshire.
Carole Middleton, Kate's mother, revealed that she has recently largely ditched animal products - and will be serving a vegan option alongside the traditional meal.
"I've recently gone vegan," Carole told The Sunday Telegraph's Stella magazine. "Or maybe that should be flexitarian.
"If I go to someone’s house for dinner, I'm not going to make a fuss.
"For Christmas, I'd probably have two options – very traditional and something vegan."
It is not known whether Kate and William will tuck into Carole's vegan offering.
The royal pair have come under fire for sharing a passion for killing animals - taking part in hunting and shooting rituals.