As David Cameron continues talks with EU leaders in Brussels on his renegotiation proposals, ITV News spoke to Richard Corbett MEP about the significance of the timings and what options the 28 leaders have to talk to on the sidelines if there are still disagreements.
Corbett, now a Labour MEP, was a political adviser to Donald Tusk predecessor Herman Van Rompuy and attended dozens of summits.
Speaking about Mr Tusk's job, Corbett said: "His job is to get a consensus out of 28 prime ministers and presidents. Not an easy bunch to deal with.
"A very difficult meeting to chair. You've got 28 prima donnas around the table who all think they are right and usually get their own way domestically. They're suddenly in a meeting where none of them can get their way until everybody around the table agrees. Very challenging."
Mr Corbett said between the opening session and the formal dinner some leaders may go into bilateral consultation - to argue out or discuss an issue with another member.
And when the leaders go to bed, their advisers can continue to discuss issues, come up with ideas and prepare draft texts into the early hours.
Mr Corbett said it could be seen as a good sign that Cameron's EU proposals were discussed in the opening session.
"It's probably a good sign that they think they can settle that before dinner. The difficult subjects are usually left to dinner because they can go on and on and sometimes late into the night and the early hours of the morning and so it's probably a good sign, unless they think they need two bites at this - one today and then one tomorrow morning."
He added: "I think everybody wants a deal to be reached, there's a lot of political will to make that happen, but remember it only needs one head of government, one prime minister or president to say 'This is not acceptable for my country' or for there to be a problem and if they don't solve that problem this time they'll have to come back to it next month."