Brendan Cox is facing his second Christmas without Jo, mother to their two children.
Extremist Thomas Mair was jailed for life for killing the Labour MP in the days before the Brexit referendum last June.
On Christmas Eve, Mr Cox wrote a message on Twitter, urging people to tell their loved ones how much they mean to them.
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."
The tweet rapidly gained traction on the social media site, racking up more than 12,000 retweets and 31,000 likes.
Mr Cox later said he "wasn't expecting" the reaction his tweet had sparked, as followers thanked him for his message and shared their own experiences of loss and heartbreak.
He wrote: "Wasn't at all expecting this outpouring of kindness to me or to each other in this thread. A lovely reminder at the end of the year that bad [things] happens but the world is still full of good people."
He also re-tweeted one follower, who had replied to his initial tweet saying: "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."