Brendan Cox, whose Labour MP wife was murdered by a far-right fanatic in June 2016, received hundreds of messages after his comments.
He wrote: “Weird thing to do on Christmas Eve but I’d encourage everyone to take a moment to imagine losing the person you love most in the world.
“Imagine what you would wish you had said to them. Then take a moment to say it.”
In response, hundreds of people told stories of losing or having lost loved ones around Christmas.
Tommy Lusk wrote: “My wife has just been diagnosed with terminal breast cancer so after reading your post I told her how much I loved her.
“I had been doing that anyway but your post was a timely reminder not to take anything for granted.”
Twitter user @Luiseach admitted: “I lost my 31 year old husband very suddenly in March this year and there is so much I wish I could still say to him. This message is so important.”
I always do this. I’ve already lost my twin sons and my mum. All we have is this moment.
— Amy Antoinette (@amyantoinette) December 24, 2017
While Fiona Paterson said: “Life is too short. My brother, whose birthday it is today, would have been 46. He died when he was 34.
“My partner and father of my son was only 29 when he died suddenly.
“A lot of people think they have time when in reality we don’t know what’s in front of us. So here’s to Love and Living.”
Others spoke of the gestures they were making now for relatives.
Writer Pete Hawkins posted: “My dad is in end of life care in a rest home. I’ve driven from Lincolnshire to Brighton to do just that.
“Had to hold his hand one last time and tell him I love him. He squeezed my finger in response. Best present ever.”
And Christine Taylor said: “My mum died in August and while I stayed with her for three months I asked her questions and listened to her.
“Already told her that I loved her every time we spoke and right up to her last breath holding her hand and kissing her #missyouMum.”
More from Christmas
Hundreds of surfing Santas take to Florida waves
Only 3% of young people will attend church on Christmas Day
Opinion: ‘Coptic Christians fear festive attacks but we will not give in’
Christmas messages: May calls for unity and Corbyn for Britons ‘to show you care’
‘Binge drinking’ Santa in need of a health check, top GP jokes
Can you do your Christmas shopping with Bitcoin?
After the posts, Mr Cox said he “wasn’t at all expecting this outpouring of kindness to me or each other”.
He described it as a “lovely reminder at the end of the year that bad s*** happens but the world is still full of good people”.