The Vale of Glamorgan Council, alongside 21 local authorities and organisation have issued a joint statement, asking residents to think carefully about what you "should do" rather than what you "can do" this Christmas.
It comes as Coronavirus is spreading faster and wider in Wales than at any other point in the pandemic, with public services under intense pressure as they work around the clock to care for and keep people safe.
The joint statement says "Thank you so much to all of you who continue to follow the rules."
"Sadly, as coronavirus cases grow even higher and coupled with the additional threat that a new variant strain of the virus can be passed on more easily, our chances of catching and spreading this awful virus also increases"
"To keep yourself and your loved ones safe this Christmas, please avoid mixing wherever possible."
"For most people the virus will cause a mild illness but there are many - our parents and grandparents - who are at risk of serious illness or death if they catch it"
"Our actions and decisions over the festive period, and in the coming weeks and months, will shape Wales for years to come."
"Please, let's all think carefully before we act and protect our public and emergency service so they are there when we really need them."
The letter has been signed by all of Wales' health boards, emergency services, councils, and the Welsh Government.