This, as passengers grow increasingly more frustrated waiting for answers.
"It's very, very hard. It's very stressful," said Diane Reyes.
The nationwide headache is sending passengers scrambling from Chicago to New York, and Las Vegas.
Lines of Southwest customers piled up at airports, like Chicago's Midway, as they try to get answers to why their flights were canceled.
"We had to get a hotel, the Ubers, food, you name it. Nothing was ever promised. They didn't comp us anything," said Lisa Harmon.
"I need to be reimbursed. I need to be said, 'We're sorry,'" added Brett Yeates.
Southwest is blaming a perfect storm of weather early in the weekend in Florida, paired with unforeseen air traffic control issues in the same region.
However, the FAA said its staffing issues ended Friday, adding that military training compounded challenges airlines have faced.
"This is just unbelievable and we're not getting correct answers," Reyes said.
As the weekend comes to an end, some Southwest passengers said employees have told them the issue is Southwest's company-wide vaccine mandate, with some flight attendants and pilots allegedly forcing network-wide cancelations in protest.
"I spoke to a pilot that was walking and I said, 'Sir, what is going on?' He says, 'I'm not getting the vaccination,' turned away from me and just kept on walking," Yeates said.
The union representing Southwest pilots denies that allegation and said its "pilots will continue to overcome Southwest Airlines management's poor planning, as well as any external operational challenges."
Southwest said it's focused on helping customers as fast as possible, adding, "We know the frustration flight cancelations are creating for our customers and employees and we apologize..."
Regardless of the cause, passengers said they're out extra money and missing important events. One man ABC7 spoke with missed his sister's wedding, where he was supposed to walk her down the aisle.
"I was playing an important role so it really touches me," Kerence Jackson Sr. said.
"We're heartbroken. It's devastating. You can't redo that," his wife added. "Most of the time if they have stuff going on, at least they try to delay you. It was just canceled - like they knew this was going to happen."
Some people also said they're missing their bags. One group said some of their luggage was sent to one airport but they've been rebooked to another destination.
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The flight-tracking website FlightAware.com shows that Southwest canceled 28% of its flights on Sunday, while 600 more face delays. More than 800 Southwest flights were canceled Saturday and upwards of 1,100 flights were delayed, according to FlightAware.
For comparison, American Airlines has the second-most cancellations Sunday with 142, or 4%, as of 4:30 p.m. ET.
The colossal travel hiccup comes in the midst of a busy time for airlines -- Monday is a federal holiday, meaning millions of American workers have a three-day weekend. More than 2.1 million people were screened at airport security checkpoints Friday, a volume that hasn't been seen since early August, according to the Transportation Security Administration.
Videos and images posted to social media show Southwest customers waiting in long lines that snake through airports in Denver, New York, Missouri and Arizona. Travelers in Chicago, Houston and other major U.S. city reported similar turmoil.
"Hey @SouthwestAir ... I'll be missing my friend's wedding because of you," one person tweeted.
Southwest's response
The Dallas-based airline tweeted Saturday that the problem stems from air traffic control issues and weather. In a more detailed statement issued Sunday, Southwest elaborated on how "weather challenges" and "unexpected air traffic control issues" in Florida on Friday created a ripple effect of nationwide delays and cancelations that spilled into the weekend.
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"We experienced weather challenges in our Florida airports at the beginning of the weekend, challenges that were compounded by unexpected air traffic control issues in the same region, triggering delays and prompting significant cancellations for us beginning Friday evening ... With fewer frequencies between cities in our current schedule, recovering during operational challenges is more difficult and prolonged," the statement said.
"Some airlines continue to experience scheduling challenges due to aircraft and crews being out of place," an FAA spokesperson said in a statement.
Southwest pointed affected customers to its self-service rebooking options on southwest.com. The airline said it is working to recover operations and thanks customers for their patience.
MORE: Southwest employees must be fully vaccinated by Dec. 8 to keep jobs, company announces
Southwest is latest airline to mandate vaccines for workers
Anti-vaccine "work slowdown" could be catalyst for cancellations, analyst says
Henry H. Harteveldt, president and travel industry analyst at The Atmosphere Research Group, based in San Francisco, points to other causes for the cancelations.
First, he said Southwest has scheduled more flights than it can handle, a problem that started in June. He also noted that Southwest operates what's known as a point-to-point route network, and when a delay occurs, it "cascades" along the remaining flight segments. That's because, for example, a Southwest flight departing Fort Lauderdale, Florida, for the airline's home base of Dallas may make multiple stops along the way.
But Harteveldt told The Associated Press the most troubling reason is the likelihood that some pilots who oppose Southwest's decision to mandate COVID-19 vaccinations are participating in an illegal job action where they call in sick or are engaging in a "work slowdown."
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ABC7 asked Southwest about the allegations tied to the vaccine mandate. The airline pointed us back to its initial reasoning.
In a statement Saturday, the airline's pilot union, Southwest Airlines Pilots Association, said that's not the case.
"SWAPA is aware of operational difficulties affecting Southwest Airlines today due to a number of issues, but we can say with confidence that our pilots are not participating in any official or unofficial job actions," it said.
Bad year for Southwest
Southwest has had a difficult year. The airline had the worst on-time performance and the greatest percentage of canceled flights of any of the nation's four major airlines in June and July, according to flight-tracking service Cirium.
Passengers aren't the only ones upset with Southwest: Unions for pilots, flight attendants and mechanics have all voiced complaints about the airline's operations.
Southwest promised to address the problems by scheduling fewer flights in the fall.
But Harteveldt noted Southwest's woes could linger and affect its fourth-quarter financial performance.
"All of this is happening as people are in the midst of booking their Thanksgiving and Christmas/New Year holiday travel," he said. "It's very possible that some people who might normally book on Southwest may see this news and choose to fly other airlines."
The Associated Press and CNN Wire contributed to this report.