It is likely to be one of the biggest protests over immigration, an issue that has proved deeply divisive under the presidency of Donald Trump, the BBC’s David Willis reports from Los Angeles.
What do the protesters want?
People across many major towns and cities in the US have been urged to gather under the slogan #familiesbelongtogether. Organisers say they want to send a message to President Trump, who they fear will go back on his executive order.
“We cannot slow down now since the court ruling alone isn’t enough and could be overturned,” the movement’s website says.
As well as reuniting parents with their children, organisers are calling for an end to immigrant detention – even when families are kept together – and also plan to voice opposition to President Trump’s travel ban targeting five majority-Muslim nations, which was upheld by the US Supreme Court earlier this week.
Much of the uproar over the separation policy came after news organisations reported children being held in cells, converted warehouses and desert tents around the country.
Who is taking part?
Organisers say they expect tens of thousands of people to turn up – with instructions to dress in white, to represent peace and unity. The main march is in Washington DC, but one of the people behind the movement, Anna Galland, says there will be separate events in 50 states in total.
Lead organisers of the march include the American Civil Liberties Union, the Leadership Conference on Civil Rights and the National Domestic Workers Alliance.
A number of celebrities have also expressed their support, including actresses Julianne Moore, America Ferrara and Natalie Portman, and Lin-Manuel Miranda, creator of the hit musical Hamilton.