Until today, you could apply at the last minute, but from now on you must apply at least 72 hours in advance to avoid the risk of being denied boarding your flight.

We are facing one of the biggest travel holidays of the year and the new rules for ESTA (Electronic System for Travel Authorisation) could have major consequences for travellers who wait until the last minute before applying for entry permits.

The US Department of Homeland Security issues and approves travelers' ESTAs. The agency has consistently advised passengers to apply at least three days in advance, but since applications made electronically are often approved immediately, travelers have been able to apply right up until they check in at the airport.

No more late applications
But now the last-minute applications are over. On its website, the US Department of Homeland Security has written that so-called ”Real-time approvals” no longer exist. ”…arriving at the airport without an approved ESTA will likely result in you being denied boarding”:

It should also be kept in mind that having an ESTA does not guarantee entry to the US. You must also be approved by a Customs and Border Protection officer on site in the US.
At the same time as the new ESTA rules are being introduced, American airports are also tightening their security over the Christmas holiday.
“Passengers may experience increased screening and additional baggage screening,” a spokesperson for the US Department of Homeland Security said.

Source: aftonbladet.se