While the government is reopening for at least three weeks, it’s unclear whether all I.R.S. employees will return to work immediately, Mr. Reardon said on Friday evening. “The agency is hopeful of getting normal operations going on Monday,” he said, since tax filing season is officially starting. But workers, he said, were awaiting specific instructions from their supervisors. The union is urging the government to be flexible and pay workers as quickly as possible, he said, since many are facing financial hardship from missing two paychecks during the shutdown.

As to the I.R.S. help line, taxpayers and tax preparers will probably have longer waiting times this year.

“Even in a good season, there’s a backup,” said Edward Karl, vice president for taxation with the American Institute of Certified Public Accountants.

Despite the uncertain environment, Mr. Karl said, taxpayers who typically file early should do so, as soon as they have the necessary documents, like W-2 income statements.

“The critical thing for taxpayers is to presume it’s business as usual,” he said.

Meanwhile, the I.R.S. has announced a bit of relief for some taxpayers: If you underpaid your taxes in 2018, you’re less likely to pay a penalty this year. The agency said this month that because of tax changes passed by Congress in the Tax Cuts and Jobs Act, it would waive the penalty for many filers who underpaid.

The change — just for the 2018 tax year — is aimed at helping taxpayers who were “unable to properly adjust” their withholding and estimated tax payments because of the “array of changes” under the tax law, and may have inadvertently underpaid, the I.R.S. said.

The federal tax system is “pay as you go,” so taxes must be paid periodically, rather than all at once. Typically, taxpayers pay a penalty if their payments, from paycheck withholding or extra estimated tax payments, total less than 90 percent of the amount owed. (Other criteria also apply. For instance, if you underpaid but owe less than $1,000, you’re not subject to any penalty.) For 2018 returns, the I.R.S. said, the threshold will be lowered to 85 percent.