Her widower Brendan said the UK-wide event, called The Great Get Together, would be a "fitting tribute" to the mother-of-two who died on 16 June 2016.
Her murder was "designed to divide our country" so uniting in this way would be "a powerful statement", he added.
The Royal Family and several charities are helping to publicise the event.
Mrs Cox, MP for Batley and Spen, was murdered outside her constituency office in Birstall, West Yorkshire, by Thomas Mair, who shouted the words "Britain first" as he shot and stabbed her.
Mr Cox previously told BBC Radio 2's Jeremy Vine his late wife "loved a party" and would have been "thrilled by the idea" of people coming together on her behalf.
"We wanted something that celebrated Jo's energy but also brought the community together," he said.
"That could be picnics on your village green, street parties, shared barbecues, community bake-offs. The basic idea is to get together and have fun."
But he said the event also sent a strong message.
"The killing of Jo was a political thing designed to divide our country and split communities apart.
"As a reaction to that event, [we will] bring the country together en masse."
Mrs Cox used her maiden speech in Parliament in 2015 to say people in the UK "have far more in common with each other than things that divide us".
The event is expected to be similar the Queen's Diamond Jubilee in 2012, when more than eight million people took part in neighbourhood street parties.
Last year, 7.3 million people held lunches in streets, gardens and community spaces as part of a project known as The Big Lunch, which began in 2009.
This year, The Big Lunch will join forces with the Jo Cox Foundation to co-ordinate events on the 17 and 18 June.
"We may even surpass the Jubilee," said Peter Stewart, director of The Big Lunch.
As well as street parties, charities including the Royal Society for the Protection of Birds, NSPCC, Women's Institute, Help for Heroes and RNLI are planning their own events.
The RPSB said it would hold activities across its network of nature reserves. with details to come in the next few weeks.
"Whether it's the Olympics, Jubilee or Royal Wedding people love that opportunity to come together," Mr Cox added.
"But being British we need that excuse to get together."
The Duchess of Cornwall and chef Jamie Oliver will join Mr Cox for the launch of the The Great Get Together at Clarence House later.

The distasteful adjective - a favourite of Great British Bake Off presenting duo Mel and Sue - has topped polls in the UK, US and Canada, according to those behind the global survey.
Oxford University Press said "no", "Brexit" and "British", joined moist as the UK's four most-despised words.
More than 10,000 people have responded to the survey so far.
"Moist" - dubbed by comedian Miranda Hart the "queen of all words" - also leads the way in the US, Canada and Australia, also coming second in the poll of Dutch opinion.
"No" is least popular in the early running in the Netherlands and Germany, while "love" and "sorry" find the least favour in India.
Dan Braddock, from Oxford University Press, said this was the first time Oxford Dictionaries - which focuses on current English usage - had tried such an experiment and "we're not entirely sure what to expect yet".
"We thought for a long time about the question to open with, but we decided to go down the negative route," he said.
"We thought people were more likely to have strong opinions about a negative subject matter."
He said the publisher hoped to collect 30,000 replies by the end of the project.
Despite the poll's findings, "moist" does not appear to be universally despised.
On Wednesday's edition of The Great British Bake Off, host Mel Giedroyc remarked that it was her "favourite word" as she urged contestants to produce a "sumptuous" drizzle cake.
Thousands of street parties, picnics and baking competitions will be held on 17 and 18 June to remember MP Jo Cox, a year after she was murdered.
[NEXT_CONCEPT]
"Moist" is the early front-runner in a search to find the most-hated word in the English language.