May 22, 2015 7:00 AM EDT

Summer travel is kicking off this Memorial Day weekend, and major airports around the county are preparing for roughly 3 million Americans who’ll board a flight. Not all of the travelers, of course, will make it to their destinations on time—some will be part of the unlucky bunch whose flights become cancelled or delayed.

The job of determining who will be on time and who won’t falls upon a small group of airport employees who make inputs into a computer program they call the Cancellator. Their goal is to preserve an airline’s original schedule as much as possible. And that’s no easy task—each time one flight is delayed or canceled, other flights using that plane become affected, too.

Want to know more about the software and employees behind your airport frustrations? Read TIME’s March 3, 2014 cover story on airline cancellations here.

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