By Alison Sider 

Flights have been packed but relatively painless during the week of Thanksgiving, the first major test since airlines took steps to resolve traffic snarls that have plagued Americans' return to air travel.

For seven days in a row, through Wednesday, daily airport passenger volumes exceeded two million people, a streak not seen since before the pandemic, according to the Transportation Security Administration. U.S. airports bustled with over 2.3 million people Wednesday -- the most hectic day since February 2020. Sunday is expected to be even busier, TSA officials have said.

Thanksgiving is always one of the busiest travel periods of the year, marked by long lines and big crowds at airports. There are additional stresses this year. Airlines have had a rocky re-emergence from the pandemic, and some carriers have been especially vulnerable to bad weather or glitches that have caused their operations to unravel for days, resulting in thousands of canceled flights.

So far, there have been few major disruptions this week. Less than 0.5% of U.S. domestic flights were canceled Monday through Wednesday, according to FlightAware, a flight-tracking site. Around 60 U.S. flights had been scrapped as of Thursday afternoon.

Sunday, which is expected to be the busiest day yet, could prove to be the next challenge.

Airlines' success so far this week appears to reflect both good luck and better planning, following their efforts to shore up operations ahead of the holiday. Southwest Airlines Co., Spirit Airlines Inc. and Allegiant Travel Co. have scaled back their flying plans in the final months of the year to better align schedules with staffing and other constraints.

Carriers have been on a hiring spree, hoping to bulk up their ranks after encouraging thousands of workers to retire or take long-term leaves of absence during the pandemic. Southwest has said it has hired over 4,500 new workers -- 85% of its goal for the year.

They have been tackling long waits for customer service and other irritants for passengers as well as making sure they have staffing reserves to provide a buffer for any upsets during the holiday season.

Airlines including Southwest, JetBlue Airways Corp. and American Airlines Group Inc. have also been offering incentives such as cash bonuses and extra pay for employees who work over the holidays. American flight attendants who have perfect attendance over the holiday period will be in line for triple pay.

In a letter to customers Wednesday, JetBlue President Joanna Geraghty said the airline had been preparing to serve a record number of travelers this holiday season. It has been hiring customer support and airport staff, streamlining lobby lines and adding signage to help people find their way in airports. The airline has also worked to cut customer service hold times to under an hour, on average, she said.

"Ramping back up has also not been without its growing pains -- and we appreciate your continued patience and support as we take this journey together," she wrote.

Thanksgiving is always one of the busiest travel periods of the year, marked by long lines and big crowds at airports. There are additional stresses this year. Airlines have had a rocky re-emergence from the pandemic, as demand roared back more quickly than expected and airlines were caught flat-footed and shorthanded as they raced to recall and retrain workers who weren't needed when travel demand dried up last year. They have often struggled to recover when things go awry, and carriers that expanded aggressively to capture rising demand have been especially vulnerable to bad weather or glitches that have caused their operations to unravel for days, resulting in thousands of canceled flights.

"I was very nervous about traveling, especially on the Wednesday before Thanksgiving," said Chloe Corcoran, who flew from Los Angeles to visit her family in Rochester, N.Y. "I feel extremely fortunate things have gone well."

While there are still several days of holiday travel left to go, airlines have had some things working in their favor. For one, the weather has been mostly calm, as early predictions of storms fizzled. Bob Larson, a senior meteorologist with AccuWeather, said there could be local weather troubles leading to travel delays in parts of the country, but more severe weather isn't in the forecast.

American learned several lessons recently when high winds slowed traffic at its Dallas-Fort Worth hub, rippling through its operation for days and leading to more than 2,000 canceled flights, Chief Executive Doug Parker said.

"It showed us what can happen if we have a really disruptive situation hit one of our airports. And we can't let that happen over the holidays, " Mr. Parker told flight attendants in a town hall last week. Mr. Parker said the airline is prepared to handle anything that might arise, and there are already signs its incentive program is paying off, with lower rates of flight-attendant absences after it went into place.

To be sure, there are still some headaches. Unlike last year, when warnings from U.S. health officials kept people at home, travelers are once again encountering the holiday crush. Airports have warned that parking lots are filling up, and the TSA has advised people to arrive early as long lines are making a comeback.

Bigger crowds have raised fears of another wave of incidents involving disruptive passengers that have led to delayed or diverted flights -- a problem that the industry has been grappling with all year. The frequency of such incidents has fallen since earlier this year, but the Federal Aviation Administration has launched over 1,000 investigations in 2021, compared with 2019 when there were just 150 such investigations.

Attorney General Merrick Garland in a memo Wednesday directed U.S. attorneys to give priority to prosecution of crimes on aircraft that endanger the safety of passengers and crews, citing concerns about an uptick in such behaviors with millions of people traveling over the holidays. The FAA earlier this month said it had referred some three dozen incidents to the FBI for investigation.

Ashley Brown was worried about crowds the day before Thanksgiving, so she opted to depart from Hollywood Burbank Airport, which is smaller and easier to navigate than LAX. Still, when she pulled up to the airport Wednesday morning, traffic was already gridlocked. The security line stretched out the door and onto the sidewalk.

"I literally got in line and prayed," she said. After a wait of about an hour, Ms. Brown said she made it to her gate minutes before the flight started boarding.

Write to Alison Sider at alison.sider@wsj.com