European budget airline Ryanair will cancel about 50 flights per day over the next six weeks after an oversight regarding pilots' 2017 vacation time.

“We have messed up in the planning of pilot holidays and we’re working hard to fix that,” Kenny Jacobs, Ryanair’s chief marketing officer, said in a statement over the weekend.

Through the end of October, Ryanair will cancel between 40 and 50 flights daily to work through a backlog of pilot vacation time. Passengers onboard these flights can either rebook or get a refund for their travel.

The mishap was caused by Ryanair changing its annual calendar. In 2018, the airline will make a switch and move the start of its year from April 1 to January 1. The Irish Aviation Authority mandates that if the airline wishes to make this switch, all crew must receive their vacation time by the end of December. The airline is canceling flights now before the busy winter travel season begins.

Customers can check online for a list of canceled flights through September 20. Ryanair said that it will release a full list of canceled flights (through the end of October) by Tuesday. The airline said that less than two percent of its flights are affected by the cancelations.

“We have operated a record schedule and traffic during the peak summer months of July and August but must now allocate annual leave to pilots and cabin crew,” spokesman Robin Kiely said in a statement. “We apologize sincerely to the small number of customers affected by these cancellations, and will be doing our utmost to arrange alternative flights and/or full refunds for them.”

Ryanair said that the cancelations would also “improve its system-wide punctuality which has fallen below 80% in the first two weeks of September.”