More than 1,260 flights had been cancelled as of Sunday afternoon, according to flight tracking website FlightAware. That followed more than 2,300 cancellations Friday and another 1,500 on Saturday. More than 350 of Sunday’s cancellations involved aircraft scheduled to fly to or from US cities.

Additionally, hundreds of flights worldwide were cancelled by mid-afternoon Sunday, adding to the mounting number of scrubbed flights during the busy Memorial Day holiday weekend in the US.

Delta Air Lines cancelled the most flights among major US airlines, with more than 250 flights, or 9% of its operations, eliminated Saturday. More than 150 Delta flights were cancelled by mid-afternoon Sunday, according to FlightAware.

Saturday’s cancellations were due to bad weather and “air traffic control actions,” Atlanta-based Delta said in an email to The Associated Press, noting it was trying to cancel flights at least 24 hours in advance of the Memorial Day weekend.

Delta announced on its website on Thursday that from July 1 to Aug. 7, it would reduce service by about 100 daily departures, primarily in parts of the US and Latin America that Delta frequently serves.

“More than any time in our history, the various factors currently impacting our operation – weather and air traffic control, vendor staffing, increased COVID case rates contributing to higher-than-planned unscheduled absences in some work groups – are resulting in an operation that isn’t consistently up to the standards Delta has set for the industry in recent years,” Delta’s Chief Customer Experience Officer Allison Ausband said in a post.

Airlines and tourist destinations are anticipating huge crowds this summer as travel restrictions ease and pandemic fatigue overcomes lingering fear of contracting COVID-19 during travel.

Many forecasters believe the number of travellers will match or even surpass pre-pandemic levels. However, airlines have thousands fewer employees than they did in 2019, and that has, at times, contributed to widespread flight cancellations.

People who are only now booking travel for the summer are experiencing the sticker shock.

Domestic airline fares in the US for summer are averaging more than US $400 for a round trip, 24% higher than this time in 2019, before the pandemic, and a robust 45% higher than a year ago, according to travel-data firm Hopper.