Monday's recall of Parliament is the sixth since David Cameron became prime minister in 2010.
In a break from the norm, MPs from different parties are to set together in a sign of unity.
Members of the Northern Ireland Labour Party and others braved the rain at noon on Sunday to sign a book of condolence.
Before a minute's silence was held, a musician in the small crowd performed a version of the Bob Dylan song, She Was A Friend of Mine.
Yesterday, Mrs Cox's husband Brendan tweeted about how his family had remembered Jo in tents under the stars.
"Jo loved camping. Last night the kids & I camped in her memory & remembered the last time we were all woken by the dawn chorus #MoreInCommon", he wrote.
Mair appeared before magistrates on Saturday, he gave his name as "Death to traitors, freedom for Britain".
Brigitte Anton of the Labour Party in Northern Ireland first met Jo Cox three years ago while working in the Labour Women's Network.
Speaking to the Belfast Telegraph at City Hall yesterday she described her late colleague as "a lovely, lovely person and a very dedicated politician."
"She was like one of the sisters, as we say. Very gentle and very kind, very authentic. I just can't believe anybody would attack her, it's just horrible to think about it."
Ms Anton said she believed the killing had been politically motivated.
"It was brought about by the poison in society, that hatred that has been going on for quite a while now, especially in political discourse. We were talking today about overcoming that hatred and making a better life for our children."
Also attending was Emma Hutchinson, the women's officer for the local party.
"I think it's possibly the first time an MP has been killed outside of terrorism for their political beliefs," she said.
"She wasn't a big hitter MP, but she worked diligently for her constituents and further afield for the Syrian children. Across the water the EU referendum has stirred up some quite unsavoury opinions and a nasty debate at times, but it's great to see people speaking from a progressive voice who won't be silenced and have come out in support of her to show she won't be forgotten."
South Belfast MP, the SDLP's Alasdair McDonnell, had known Jo Cox briefly during her first year in Westminster.
"Thursday was stunning for all of us," he said. "While MPs do suffer a certain amount of abuse and antagonism, to have somebody shot dead is quite frightening and intimidating for all of us. Jo was a tremendously passionate individual in the issues she took up, in particular Syria and the refugees."
He continued: "We have a particular solidarity here in Northern Ireland as MPs lost their lives."
Dr McDonnell noted that in his own constituency of South Belfast, a predecessor, Robert Bradford - a Vanguard Unionist MP killed by the IRA in 1981 - was shot dead, and 1982 Assembly member Edgar Graham of the UUP was also gunned down a year after the election.
"We've had a sense of this and the pressure MPs are under, but no matter how and when it happens it's not acceptable and can never be justified."
In the village where she was killed, prayers were said yesterday at St Peter's Church for Jo Cox and her family.