The Internal Revenue Service will offer personal telephone assistance today and tomorrow and longer office hours Monday to help taxpayers scrambling to meet the midnight filing deadline.

"Taxpayers can telephone for personal assistance from 1 p.m. to 5 p.m. Saturday and Sunday," said IRS representative Rod Young. He warned, however, that weekend telephone assistance is designed only for those who need help completing their return; the IRS staff won't answer other kinds of questions, such as queries about refunds, this weekend.

Washington area taxpayers living in Washington and the Maryland suburbs can dial 488-3100. Those living in Northern Virginia can call 557-9230.

In addition to the telephone service, the IRS has a 24-hour Tele-Tax service that provides taped messages containing tax information about specific topics such as who can use income averaging, how to itemize contributions and who qualifies for child care credit. Some topics are also available in Spanish. For the list of tapes and the telephone numbers for your area, check the tax instruction book.

Here are the addresses for the offices that will be open until 6:30 p.m. Monday:

* Washington: 1201 E St. NW, Room 703 for information and Room 803 for forms.

* Maryland: Penn-Silver Office Building, 5408 Silver Hill Rd., Room 204, Forestville (formerly Suitland) and Wheaton Plaza South, lower level, Viers Mill Road and Georgia Avenue.

* Virginia: Baileys Crossroad, 1 Skyline Pl., 5205 Leesburg Pike, 9th floor.

If you need tax forms and instruction booklets when IRS offices are closed, you may be able to find what you need in a public library or post office lobby.

To deal with the traffic that inevitably develops on the night of the filing deadline at the main U.S. Post Office, North Capitol Street and Massachusetts Avenue, the Post Office plans to have 12 uniformed employes stationed on the streets starting at 7 p.m. Monday to accept tax return envelopes from motorists.

Before mailing your return, however, the IRS urges that you allow enough time to recheck the math.

"Last-minute math errors are common because people are in a rush to get in their returns by the due date," Young said. "So we recommend that after they finish, that they set the return aside for a while and then go back over the math."

Other IRS tax tips: Check to make sure the return is signed. Use the label and envelope provided with the tax package mailed to your home. Make sure you read the tax table correctly.

Taxpayers who can't meet the filing deadline can get an extension by filing Form 4868, entitled "Application for Automatic Extension of Time to File U.S. Individual Income Tax Return." That form, which must be filed by midnight Monday, should be mailed to the same tax service center that would handle your tax return.

While the extension allows more time for filing the return, it doesn't allow postponement of tax payments. A payment of estimated tax owed must be filed with the extension by midnight Monday, and the estimated payment must be within about 10 percent of the amount owed to avoid a penalty. For instance, if a taxpayer estimates that he owes $100 and submits a payment of $100, he faces a penalty if the return shows that he owes more than $110.

Finally, don't forget that the new IRS rules require that 1984 IRAs (Individual Retirement Accounts) be purchased no later than Monday. "Unlike previous years," Young said, "the IRA must be funded by midnight Monday if you want to claim it on your 1984 return."