Forgetting to turn on the oven for the Christmas turkey could be a sign of early dementia in a loved one, says the NHS's top dementia expert.
Prof Alistair Burns said becoming confused in a strange house and forgetting relatives' names may also be early signs of the disease.
He said it was important to look for changes in normal behaviour in older family members.
There is usually a rise in calls to the Alzheimer's Society in January.
The charity said this was because many people had seen relatives at Christmas gatherings and wanted advice on how to broach the subject.
Prof Burns, NHS England's national clinical director for dementia and older people's mental health, has drawn up a list of dementia signs:
- disorientation or confusion when in a new place or new environment
- forgetting the names of loved ones to the extent that it causes embarrassment
- forgetting to buy someone's present
- forgetting to cook things for a big Christmas dinner, forgetting to switch the oven on or cooking things in the wrong order
'Slip by unnoticed'
Around 850,000 people have dementia in the UK, and it mainly affects people over 65 - although it can develop earlier.
It is estimated that one in three of us will care for someone with dementia at some point in our lives.
Prof Burns said: "Dementia is something that happens slowly so it may slip by unnoticed in people we see regularly.
"That's why the Christmas visit to wider family and friends is an opportunity to spot the early warning signs."
He urged everyone to take time to consider whether someone they know may need help.
Prof Burns also said a visit to a relative or neighbour who might be alone could make a huge difference to their mental health, particularly if they were lonely.
Broadcaster Fiona Phillips, whose parents both had early-onset dementia, said Christmas was a "huge benchmark" in spotting the symptoms.
"I spent every Christmas with my parents," she told BBC Radio 4's Today programme.
But one year, "we got there and I was absolutely staggered - there was no tree up, both of them were very, very stressed".
Ms Phillips, an Alzheimer's Society ambassador, said her mother showed signs of dementia in her 50s.
But she noticed a change that Christmas - when her parents bought her a fluffy toy and her brother an orange ladies' jumper.
"We went into a horrible scene," she said. "We knew things weren't right."
Erika Aldridge, from Alzheimer's Society, said: "It can be difficult to know how to discuss concerns with a loved one, and there is no right or wrong way to approach this.
"Play a board game instead of watching another hour of TV, get up and take the dog for a walk or go for a family ramble instead of snoozing in the chair for an hour."