The agency issued new advice on Wednesday, warning people should avoid gathering inside and limit the number of contacts they had.
Regarding celebrations, the WHO's guidance is tougher than the UK's, which allows people to attend places of worship in all tiers.
The WHO suggests “religious processions, holiday markets and church celebrations” should be postponed or considerably downsized.
If they do happen “they should be held outdoors whenever possible or be limited in size and duration, with physical distancing, ventilation, hand hygiene and mask use, as appropriate.”
Watch: PM wants people to have a smaller Christmas
The WHO warns “indoor gatherings, even smaller ones, can be especially risky because they bring together groups of people, young and old, from different households, who may not all be adhering to the same infection prevention measures.”
It adds “gatherings should be held outside if possible, and participants should wear masks and maintain physical distancing.”
If held indoors, it advises “limiting group size and ensuring good ventilation to reduce exposure risk are key.”
Travel restrictions will be lifted across the UK for five days between December 23 and 27 to enable people from different households to gather over the holiday.
Up to three households will be able to come together to form a ‘Christmas bubble’ during that period.
Members of a bubble can meet at home, in places of worship and in outdoor public places.
Boris Johnson suggested those in vulnerable categories should wait until they are vaccinated before they get together with family and friends.
But the WHO warned, “vulnerable people and older friends or relatives may find it very difficult to ask loved ones to stay away physically, regardless of the anxieties or concerns they may have.”
It urged people to consider this before they made their plans this holiday.
The WHO added people who were travelling during the next few weeks should also avoid any transportation that might be crowded.
A top lawyer has claimed the UK’s Christmas COVID laws allow “almost unlimited indoors mixing between households”.
Human rights barrister Adam Wagner explained relaxed regulations allowed mingling significantly further than the three households stated.
He said the law would extend this limit as each household was allowed to bring a “linked household”.
Watch: The COVID dos and don’ts of Christmas this year