With ninety percent of their personnel considered essential, the Department of Homeland Security will be hit the hardest. Nearly 54,000 Customs and Border Protection agents and 42,000 Coast Guard employees are projected to work without pay, and as travelers flood the nation’s airports and train stations, 53,000 T.S.A. agents will keep working, as will air traffic controllers and aviation and railroad safety inspectors.

Law enforcement officers at the Justice Department will also be expected to continue working over the holidays without pay, including nearly 17,000 correctional officers, 14,000 F.B.I. agents, and 4,000 Drug Enforcement Administration agents. And after a long year battling ferocious wildfires, about 5,000 firefighters with the U.S. Forest Service will also remain on duty.

Congress passed five spending bills earlier this year totaling nearly $900 billion of the $1.2 trillion in federal agency operating expenses. Those bills funded the departments of Labor, Energy, Defense, Health and Human Services, Education, Veterans Affairs and the legislative branch — so they will remain unaffected. Mandatory spending programs like Social Security and Medicare also will continue.

The status of National Parks will be up in the air.

With funding for the Department of the Interior slated to expire, guidance issued by department officials states that services at all of the country’s 58 national parks would shutter over the holidays, including the parks’s visitor centers and restrooms. Park staff would be furloughed, although the parks themselves would remain accessible.

However, some parks, including the Grand Canyon, are planning to stay open. Arizona Governor Doug Ducey announced on Friday morning he would invoke a plan developed after the January 2018 shutdown that uses state funds to keep the park accessible.