MIRAMAR, Fla. (AP) — Spirit Airlines has scheduled a shareholder vote on JetBlue’s proposed $3.7 billion acquisition of Spirit for Oct. 19.

Investors who owned Spirit shares on Monday will be eligible to vote, the airline said in a securities filing.

The sale would still need approval from U.S. antitrust regulators, a process that is expected to take several months.

Spirit’s board approved a sale to JetBlue on July 28. Spirit leaders preferred a merger with Frontier Airlines, but that stock-and-cash deal, worth far less, failed to gain enough support from Spirit shareholders.

JetBlue leaders believe that buying Spirit will give their airline enough heft to be a more formidable challenger to the dominant U.S. carriers: American, United, Delta and Southwest.

The addition of Spirit’s fleet would result in New York-based JetBlue having around 450 planes and make it the fifth-largest U.S. airline.

Spirit shares were nearly unchanged at $22.93 in afternoon trading Tuesday, well below JetBlue’s offer of $33.50 per share plus a ticking fee to cover the time regulators need to review the deal.

Spirit started as a charter operator and now operates as a so-called ultra-low-cost carrier that has low fares but adds more fees than conventional carriers. It is based in Miramar, Florida.