Thousands of flights have been canceled or delayed over the Juneteenth holiday weekend.

By the numbers: Some 19,000 flights have been canceled or delayed since Thursday, while over 4,200 U.S. flights were delayed and 900 canceled on Sunday alone as Americans marked Father's Day amid airline staffing shortages and extreme weather that upended schedules, per USA Today.

Where it stands: New York City's LaGuardia, John F. Kennedy International and Newark Liberty International airports, along with Hartsfield-Jackson Atlanta International Airport and Chicago O'Hare International Airport, were among the worst affected on Sunday, according to flight-tracking site FlightAware.

Between the lines: James Ferrara, co-founder and president of global host travel agency InteleTravel, told USA Today shortages in skilled positions such as pilots is "really what's driving" the problems airlines have faced including this weekend's "travel Armageddon."

• The Federal Aviation Administration requires pilots to retire at age 65 and thousands of pilots chose early retirement at the beginning of the pandemic. Pilots' unions say they haven't been replaced quickly enough.

The bottom line: "We're in a boom time for travel. We're blowing away all records of all previous years. So you've got this surge in demand, and you've got limitations on staffing," Ferrara told USA Today.

What to watch: Transportation Secretary Pete Buttigieg told AP on Saturday his department will monitor what happens during the July Fourth holiday weekend and may take enforcement actions against airlines that do not abide by consumer-protection standards if flight disruptions continue.

Flashback: Memorial Day airline hell was a painful preview of summer