Thanksgiving will test whether airlines have done enough to insulate their operations from shocks that have roiled air travel in recent months.

The Transportation Security Administration said it expects Thanksgiving travel to come close to pre-pandemic levels, with 20 million people likely to pass through U.S. airports over a 10-day period. Airlines including Delta Air Lines Inc. and United Airlines Holdings Inc. said they expect the Sunday after Thanksgiving to be the busiest day since the start of the Covid-19 pandemic.

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Thanksgiving will test whether airlines have done enough to insulate their operations from shocks that have roiled air travel in recent months.

The Transportation Security Administration said it expects Thanksgiving travel to come close to pre-pandemic levels, with 20 million people likely to pass through U.S. airports over a 10-day period. Airlines including Delta Air Lines Inc. and United Airlines Holdings Inc. said they expect the Sunday after Thanksgiving to be the busiest day since the start of the Covid-19 pandemic.

The jump in travelers puts pressure on airlines to deliver a smooth experience after months of stumbles. Short-staffed carriers have been more easily knocked off course. They have struggled to recover from bad weather and other disruptions that they previously might have taken in stride, analysts and consultants said, resulting in dayslong cascades of cancellations and snarling travel for thousands of passengers.

“The network is brittle,” said Samuel Engel, senior vice president of aviation at consulting firm ICF. “With the slightest slip, the airlines are showing they have limited ability to accommodate disruption.”

Nine of the largest U.S. airlines canceled 1.9% of flights from June through the end of October this year—a slight uptick from the nearly 1.6% they canceled during the same period in 2019, according to figures from Cirium, an aviation data provider.

Videos shared online show crowds of people at airports across the country after American Airlines canceled hundreds of flights over the weekend due to bad weather and staff shortages. Screenshot: WSOC-TV via AP The Wall Street Journal Interactive Edition

Frustratingly for travelers, there have been more days of acute misery during that time frame. There were 147 days when these airlines canceled 5% or more of their flights, compared with 85 such days in 2019, according to an analysis of operational data by Brett Snyder, who writes the Cranky Flier blog and runs a concierge travel service.

Airlines have been taking steps to try to address operational problems in time for the holiday surge in demand.

Southwest Airlines Co. and Spirit Airlines Inc. tempered ambitious plans, scaling back the number of flights from what they had previously planned to offer and keeping more employees on call.

American Airlines Group Inc. is paying hefty premiums to workers to make sure it can staff flights, including $1,000 bonuses for some workers and as much as triple pay for flight attendants with perfect attendance. Chief Executive Officer Doug Parker has said the carrier is well-prepared in terms of staffing.

Southwest is offering its frequent flier points to certain workers for shifts over the holidays, which they can redeem for travel. JetBlue Airways Corp.

is also offering an extra $1,000 to qualifying flight attendants who don’t call out from work through early January. While flight attendants are encouraged to take time off if they’re sick, that will require a doctor’s note, according to a message to flight attendants.

“Our customers have been telling us they’re ready to travel and they are counting us to deliver,” Ed Baklor, the airline’s head of customer care, wrote in the message.

Even with recent cutbacks, airlines are planning busy schedules around Thanksgiving. They are offering 19 million domestic seats the week of the holiday—up 46% from last year and within 9% of 2019 levels, according to data provider OAG.

Airlines were expected to have been able to ramp up quickly after the pandemic because of billions of dollars of federal aid that covered workers’ salaries until the end of September 2021, but it hasn’t worked out that way.

Fearing that the pandemic would outlast government funds, carriers encouraged thousands of employees to leave or take extended leaves of absence last year. Then demand surged in the spring, and bringing workers back and hiring new ones proved to be a slower, more complex process than carriers anticipated, executives have said.

The missteps irked some lawmakers, including

Sen. Maria Cantwell (D., Wash.), who is looking to hold a hearing on airlines’ operational problems.

The results have been an exhausting stretch for workers who have remained on the job, and the issues exacerbated disputes with labor unions while many are negotiating new contracts, traditionally a period of heightened tensions.

Unions at some airlines said management teams haven’t gone far enough to fix underlying problems that could reemerge over the busy holiday season, particularly if bad weather or technical glitches again trigger disruptions.

American’s pilots, for instance, rejected the airline’s offer of premium pay for holiday flying, saying management was throwing money at a problem without solving it. American executives responded in a letter to pilots that they were disappointed and would continue to look for ways to provide support over the holidays.

The union that represents Southwest’s flight attendants said the frequent flier miles the airline is offering aren’t enough. Southwest’s pilots said management has been too quick to blame sick calls and absences for its problems.

“The truth is that mismanagement—primarily in long-term planning and short-term daily execution—feeds the dysfunction, not pilot sick calls,” the Southwest Airlines Pilots Association wrote to its members earlier this month. “The operation is a catastrophe waiting to happen.”

A Southwest spokesman declined to comment on the unions’ remarks but said the airline has reduced flight schedules in November and December to bolster reliability and align with expected staffing, in addition to offering incentives for those who work shifts during the holidays.

A spokesman for Spirit Airlines, which canceled more than 2,800 flights in a 10-day meltdown in August, said its flying reductions have been working, boosting its place in on-time performance rankings last month.

Problems extend beyond staffing levels. Maintenance work also backed up during the pandemic. Supply-chain bottlenecks make it harder to obtain parts, and retirements among the most experienced technicians mean complicated work is taking longer, leaving fewer aircraft available, said ICF’s Mr. Engel.

In addition, tensions in airplane cabins have run high this year, with a sharp rise in unruly—and sometimes violent—passenger behavior. Rates of such incidents have fallen in recent months ahead of the holiday travel season.

Airline networks also changed during the pandemic. Carriers shifted to fly more to popular leisure destinations and thinned schedules in cities that had been business-travel hot spots. Those changes put new stresses on airports that saw heavier traffic than they were used to handling, something Allegiant Travel Co. CEO Maurice Gallagher Jr. described last month as “leisure destination overload.”

Allegiant canceled nearly 4% of its flights from June through October, and 63% were on time, according to Cirium. Mr. Gallagher said an influx of travel from other airlines has strained resources at the vacation-destination airports that Allegiant focuses on.

“At this point, the main thing is to make sure we don’t over-schedule the airline, and we have sufficient pilots, flight attendants, maintenance can do their job,” Mr. Gallagher told analysts and investors in October.

United CEO Scott Kirby said this week that some of his rivals left themselves no margin for error in their zeal to fly full schedules. Executives tasked with generating revenue appear to have overruled those charged with keeping operations running, he said.

“One hundred percent of the time when airlines have an operational meltdown, it is because the commercial team got out over their skis and didn’t listen to the operations team,” he said. “If you let them run wild, they’ll run the airline off a cliff.”

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