A poll has revealed that there is confusion among the majority of patients about who to call when their GP surgery is closed.
Of the 2,000 people surveyed by Which?, more than half were unaware they could phone an out-of-hours GP service.
The consumer watchdog is advising people to contact NHS Direct in England and Wales or NHS 24 Scotland for help and information, and suggests that details of out-of-hours GP services should also be recorded on surgery answering machines.
This weekend, many GP surgeries will be closed for a four-day stretch, with Christmas Day and Boxing Day falling just before the weekend, when surgeries are typically shut.
Nikki Ratcliff, head of services research at Which?, said: "Nobody likes to think about the possibility of getting ill or getting injured at Christmas, but unfortunately it can happen, so it's best to be prepared.
"Make sure you know what services are available in your area, what they're for and how to access them so that if you do need to seek help, you know where to go."
A spokeswoman for the Department of Health said: "In almost every part of the country, more than half of GPs (65%) are now providing early morning or evening opening, giving better access to patients.
"In addition, there are around 100 walk-in centres across the country seeing over two million people annually.