A clash over dates led to a behind-the-scenes row between Simcha on the Square organisers and Hackney Limmud, an event held for the first time this year.
Both events were held on Sunday. But the Hackney group said they had selected the date first about 18 months ago and put it on the communal calendar run by the Board of Deputies. Simcha on the Square, meanwhile, initially chose the same day, moved to a date in October and then changed back again to September 14.
One Hackney Limmud organiser said: "We were very happy when they moved their date. But when they changed their minds again, they didn't bother to tell us. Even if they didn't know the first time, by then they knew what date we had booked because some people, who wanted to take part in our day, told us they had booked for Simcha as well.
"That caused us all sorts of organisational problems. We had to change our running order because some of our musical performers had to go to Simcha in the afternoon, so we had to schedule them in the morning. We tried to produce a balanced programme across the day but that made it very difficult. It created a scheduling nightmare.
"We're a charitable bunch of people, so at first we said it must be a mistake that they chose the same day, and we'll just have to cope with it. It was just a ghastly coincidence and we were very relieved when they moved to a date in October.
"But then they moved it back and a lot of people were really upset with them. It forced some people to make a choice."
In response, Geraldine Auerbach, the festival's director, said: "Simcha on the Square is offered dates by the GLA to fit in with their very busy schedules and other events on the square.
"It took a while this year for the new mayor to settle in and for us to hear if the event would go ahead. When it was finally confirmed, the date of September 14 was the only date possible. We are sorry about the clash with Limmud.
"We hope that each event added something special to the cultural vibrancy of London."