To you, perhaps, today is the day after Christmas, when the holidays start winding down.

But to the Internal Revenue Service, it's only the beginning -- the beginning of another tax filing season, during which it will collect most of the $1 trillion it wrings out of the U.S. economy.

And in the spirit of that season, the tax collectors are dropping a little present in the mail for you. It's your 1990 tax return package -- the forms and instructions you need to calculate your taxes and pay them or claim a refund if you are due one.

If all goes well, you should have your forms by Friday.

That's a little later than usual. In the past, the IRS had been mailing returns so they arrived Dec. 26, but after some discussion officials concluded that letting another day or two pass would be a bit more tactful.

But it didn't work perfectly in the Washington area, where some happy taxpayers reported receipt of mail from the IRS before Christmas.

Most of what is in the package has a familiar look. The IRS checks each taxpayer's return from the previous year and tailors the new packet accordingly.

This means that if your financial situation hasn't changed greatly from 1989, you should get exactly what you need.

For example, if you had only wages last year and were able to use the relatively simple Form 1040A, you will get a 1040A again this year.

About 97 million taxpayers will receive these packages this year. Another 11 million, who had farm or business incomes and who used professional tax preparers, will receive a postcard because the preparers usually provide the forms. However, the postcard can be used to request a package.

The IRS figures this shortcut will save the agency $1 million in printing and postage costs.

In all, the service expects to receive 113.4 million individual tax returns, about the same as last year.

The best news for most people, though, is that Congress left the tax law largely alone for this year. The changes resulting from the budget battles of this fall become effective in 1991 for the most part, so the experience of doing last year's taxes will actually be useful this year.

"The neatest thing we can say about the tax forms for 1990 is that there are not many changes," said Arthur Altman, head of the IRS branch that designs and publishes the forms.

But there are some changes done, the IRS hopes, in the name of improvement.

Most significant is an expansion of the 1040A, a form meant for people who do not itemize their deductions and have less than $50,000 in income, Altman said.

In the past, the 1040A had no provision for retirement income, so pensioners, even though their personal finances were quite simple, had to use the long 1040 form.

For 1990, the agency has added the blanks necessary to include simple pension income -- regular pension and annuities, individual retirement account distributions and taxable benefits from Social Security.

The service also inserted a line allowing eligible taxpayers to claim the tax credit for the elderly or disabled, and it put in a line to include estimated tax payments.

As a result, 4.5 million taxpayers who had to use the 1040 last year will be able to use the 1040A for 1990.

Altman said the service has sorted out those 1040 filers who could have used the new 1040A and will send them the 1040A this year.

There also is for the first time a separate package for the Form 1040EZ, the simplest individual income tax return form offered by the IRS.

This one-page form can be used only by single taxpayers with no dependents and taxable income of less than $50,000, including no more than $400 in interest. Until now, the 1040A and 1040EZ packets had been combined.

Altman urged taxpayers to take particular care in filling out their returns. He said the IRS can correct many errors itself, but there are some important ones it cannot detect and they can cost the careless taxpayer money.

For example, if the taxpayer is 65 or older and fails to take the more generous standard deduction to which the elderly are entitled, the IRS will correct that -- but only if the taxpayer has checked the box telling the IRS that he or she is 65 or older.

If that box is not checked, the IRS will not know that the taxpayer is eligible for the higher deduction and the taxpayer will simply pay more tax than he or she should have.

Altman said 1989 data indicate that taxpayers continued to have trouble with the earned income tax credit, which can be worth up to $953 to some low- to moderate-income working parents.

To try to ease the calculations, the IRS has included in the 1040 and 1040A instructions detailed work sheets that walk the taxpayer through the qualifying steps.

The IRS will figure the credit for you, if you prefer. However, you must go through the steps to see if you qualify.

If you qualify, write "EIC" in line 28c of the 1040A or line 57 of the 1040 and the IRS will do the rest.

The IRS would like very much for taxpayers to use the mailing label that comes on their package.

"There's this myth that if you use the IRS label bad things will happen to you," said Altman. But the opposite is true, he said.

If the taxpayer forces the IRS to enter all the data manually, the likelihood of error increases substantially, and the processing of the return is slowed. Because about three-quarters of all individual taxpayers get refunds -- $900 on average -- delay is not in most people's interest, he said.

In fact, Altman urged taxpayers who want to speed up the process to file electronically. About 4 million did last year, and most got their refunds in two weeks, he said, compared to five to six weeks for paper returns. However, only professional return processors can send returns in electronically, and many charge a fee for the service.

Taxpayers who do not receive the forms they need can get them a number of ways. Banks and libraries will have the common ones. And most libraries have a big book with all the forms, which can be taken out and photocopied.

Forms can also be obtained in person at local IRS offices. They can be ordered by mail -- there is a form in the instructions material accompanying the 1040, 1040A and 1040EZ, or you can call 1-800-TAX-FORM (1-800-829-3676) from 9 a.m. to 5 p.m. weekdays and from 9 a.m. to 3 p.m. Saturdays.

For other tax questions, call 1-800-829-1040 in the District and most of Maryland and Virginia.

In Baltimore the number is 301-962-2590 and in Richmond it is 703-649-2361.