The Chicago Teachers Union has decided to walk off the job for one day on April 1, an effort meant to draw attention to ongoing contract negotiations and city schools’ dire financial outlook.

The one-day strike means that the city’s 400,000 students will miss class on two consecutive Fridays: They were already scheduled to be out of class on March 25, when city schools close for the first of three furlough days meant to save the cash-strapped district approximately $30 million.

The nation’s third-largest school district faces a $1.1 billion structural deficit, largely due to growing pension payments, and its financial troubles have given Illinois Gov. Bruce Rauner (R) an opening to push for state takeover.

Chicago Public Schools officials called the union’s walkout illegal, a breach of labor rules that they say prohibit teachers from striking any earlier than mid-May.

“Chicago’s students need their leaders to work together to convince Springfield to provide meaningful revenue reforms — not go on strike,” schools chief executive Forrest Claypool said in a statement, adding that he was “particularly disappointed that the CTU leadership has given Governor Rauner more ammunition in his misguided attempt to bankrupt and take over Chicago Public Schools.”

The CTU says that the strike is legal because the district engaged in an unfair labor practice when it decided not to award salary increases based on education and experience. (The union complained to the Illinois Educational Labor Relations Board, which has declined to force the school system to change course, but the case is ongoing.)

Union leaders, who have pledged to shut down downtown Chicago with a massive rally on April 1, described the one-day strike as an effort to press for increased education funding from the state — and for a fair contract. The union’s 27,000 members have been working without a contract since the last one expired in June 2015, and negotiations with the district have proceeded in bitter fits and starts.

“We cannot stabilize the school system, win improvements or even secure a strong contract if CPS does not have new revenue. The district is combing through school budgets looking for money in order to make the $680 million pension payment on June 30—the equivalent of trying to make a mortgage payment by looking for spare change under seat cushions,” the union said in a statement Thursday. “We are shutting the schools down for a day so we can keep them open in the days to come.”

Fast-food workers pledged to join teachers in rallying in the streets.

“This house is ready and united,” CTU President Karen Lewis said at a news conference Wednesday evening, according to the Chicago Tribune. “People are very, very upset and want to do something serious.”

The union’s governing body, called its house of delegates, was far from unanimous, voting 486-124 in favor of the move on Wednesday evening.

Chicago Public Schools officials said they will provide additional details for parents about arrangements for April 1. They warned that teachers who do not show up to work that day will forfeit their pay. That could create a savings of up to $10 million, they said, but the district may end up paying for alternative programs for students during the day.

Before the union’s vote on Wednesday, Chicago Mayor Rahm Emanuel had urged its leaders to reject a strike.

“The leadership of the union should be at the negotiating table,” Emanuel said, according to the Associated Press. “Our kids should be at their desks in the classroom and our teachers should be there giving them the essential education that they chose as a profession — not a job.”