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Just hours after a continuing resolution passed to reopen the government following the longest shutdown in U.S. history, President Donald Trump sent out a series of tweets about his border wall deterring migrants from Central America, the indictment of former adviser Roger Stone and, of course, Hillary Clinton's emails.

But as the government's reopening grabbed headlines, Trump noted among his myriad tweets Saturday that time to resolve the dispute over his border wall was already ticking away.

"21 days goes very quickly. Negotiations with Democrats will start immediately. Will not be easy to make a deal, both parties very dug in," Trump wrote.

21 days goes very quickly. Negotiations with Democrats will start immediately. Will not be easy to make a deal, both parties very dug in. The case for National Security has been greatly enhanced by what has been happening at the Border & through dialogue. We will build the Wall! — Donald J. Trump (@realDonaldTrump) January 26, 2019

As the government resumes normal operations for the next three weeks at least, federal employees will receive back pay, museums and national parks will reopen and the Internal Revenue Service, which handles tax returns, will return to full force.

On Friday, a White House official told NBC News that the Trump administration would take steps to ensure back pay as soon as possible to 800,000 furloughed employees who went unpaid during the 35-day shutdown.

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Administration officials said payroll can vary by agency and encouraged employees to contact their specific employer for further information.

Congress passed a bill earlier this month ensuring government employees would receive back pay at “the earliest date possible” once the government reopened. This would happen regardless of the next schedule payday, according to the bill.

In a statement, The National Federation of Federal Employees President Randy Erwin said he was relieved that an agreement to temporarily reopen the government had been reached, but chastised the impasse as a "an expensive political stunt" that caused irreparable harm.

"Federal workers and others have resorted to selling their possessions, and many have defaulted on loans and mortgages in order to afford heat, medicine, and food," Erwin said.

Furloughed federal workers will return to government agencies like the IRS, but that doesn't mean operations will run smoothly right away.

The disruption caused by the shutdown and the recent absence of a large contingent of IRS workers means tax returns could be delayed, according to the Associated Press.

Monday marks the beginning of tax season, but in addition to the 35-day shutdown, the IRS, which has been starved for funding for years, is also burdened by the complexities of new tax law.

“Just because you reopen the government, doesn’t mean that on Day 1 everything is normal,” Jorge Castro, a former counselor to the IRS commissioner and senior counsel to the Senate Commerce Committee who is now at the law firm Miller & Chevalier, told the AP. “There’s still a backlog. The IRS has not been at full capacity in its operations for over a month.”

While government employees were given the green light to return to work, museums and parks also announced that they would soon return to full operations.

The Smithsonian museums and the National Zoo announced in a tweet that they would open their doors on Jan. 29.

A statement from the National Park Service said that, while many parks had been accessible during the shutdown, the service said it was preparing to resume regular operations nationwide, adding that "the schedule for individual parks may vary depending on staff size and complexity of operations."

"We are grateful to have the dedicated men and women of the National Park Service back at work, serving the American people and welcoming visitors to their national parks," the statement read.