(WASHINGTON) — House Speaker Mike Johnson says he has a plan to keep the government open, at least for now. But if the rest of Washington doesn’t agree, a sizable portion of the federal government will grind to a halt come Saturday morning at 12:01 am EST — just in time for Thanksgiving and the winter holidays.

While there is plenty of reason to think a shutdown might not happen — at least not until January — here’s what would happen next if Johnson’s plan falls apart:

National parks and federal museums could close during one of the busiest times of the year

The most immediate impact of a government shutdown ahead of the Thanksgiving holiday would be the eventual closure of federally run museums and parks during one of the most popular times for tourists to visit.

The National Park Service oversees some 425 areas across the country, including parks like Yosemite and Rocky Mountain National Park, as well as historic monuments and other sites. It’s not immediately clear when each site would run out of money with it being possible that some locations can use leftover money to carry them through a few days.

According to the agency’s shutdown plan, open-air sites will mostly remain accessible to the public if the government runs out of money. But it’s likely that in other places visitors could find locked gates, closed visitor centers and shuttered restrooms as thousands of park rangers are sent home without pay.

The only work done by NPS will be to preserve and protect land, such as responding to fires or criminal activity.

A closure would be particularly problematic for tourists in Washington, D.C., where a network of federally run museums and the Smithsonian’s National Zoo are supposed to remain free and open to the public every day of the year except Dec. 25.

Thanksgiving and that holiday weekend is among the busiest times for the museums.

A closure can hurt local economies, too, where food vendors and shops depend upon holiday foot traffic. In Utah and Arizona, the governors have promised to use state money to keep their parks open in the event of a shutdown to spare local businesses from losing customers.

A shutdown could snarl airline travel, particularly if it lasts into December

If the government shuts down, 3.5 million federal workers will have to go without pay. Many of them, including some 50,000 airport security officers and 13,000 air traffic controllers, will be required to come to work anyway because their jobs are considered crucial to the nation’s security.

Federal contract jobs will dry up, too, forcing lower-income workers like janitors, security guards and food servers to be laid off by their private employers until the government reopens. Lawmakers will continue to get paid, although their staff won’t.

Criminal proceedings will continue in court, reliant on federal workers willing to show up without pay, although civil proceedings will be delayed.

In the last shutdown that stretched into 35 days under President Donald Trump, trash piled up around Washington and federal workers began calling in sick, including at airports resulting in long lines for travelers nationwide. Union officials say that without pay, many of these workers couldn’t afford to pay for child care or to fill their gas tanks to get to work.

If there is a silver lining to the timing of this latest shutdown threat, it’s that many federal workers aren’t scheduled to miss their first paycheck until after Thanksgiving, around late November or early December. And because federal workers qualify for back pay, it’s possible Congress can resolve any last-minute hiccups before federal workers — including airline employees — feel the crunch.

On the other hand, contractors don’t qualify for back pay and could immediately feel the impact of lost pay. In the last shutdown, food banks saw a surge in needy families because of lost pay.

And, if a shutdown drags into December, it’s possible that airline and other essential federal workers will begin calling in sick a soon as they miss their first paycheck, either out of protest or financial necessity.

Social Security, Medicare and Medicaid payments will continue but services could be slow

While government shutdowns are a big deal impacting every thing from border security to military pay, they actually only affect 27% of total federal spending that is up for debate every year in Congress.

These annually funded programs, known as the government’s “discretionary spending,” include disaster-relief money that might help a community rebuild after a tornado and food aid for moms and their infants. The nation’s military and space programs also are paid for through this pot of money.

But the biggest portion of federal spending is considered “mandatory” and will actually remain untouched, including payments by Social Security, Medicare and Medicaid.

While government officials say these payments generally won’t be affected, some related services could become slow such as receiving replacement cards and benefit verification services.

The U.S. Postal Service, which uses its own revenue stream, is not affected by a lapse in government funding.

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