A late Christmas card from your procrastinating Uncle Harold won't be the only thing missing from your mailbox Wednesday. Your annual greetings from the Internal Revenue Service won't be in the mail either.

The IRS, after years of sending its tax packages out on the day following Christmas, has decided to give taxpayers a brief reprieve.This year, the forms will go out on Friday, Dec. 28, three days following Christmas.

"We want to move it away from the Christmas holiday to allow folks to enjoy that period of the year as long as they possibly can before we throw a wet blanket on them," said Arthur Altman, director of the IRS tax forms division.

The packages, which cost the government $29 million to print and mail, have been waiting at local post offices for weeks and a few taxpayers may have already received theirs from eager-beaver letter carriers who disregarded the Dec. 28 date.

However, the bulk of the packages will be distributed in the period from Friday through the first week in January.

The tax forms, for the second year in a row, will feature few changes, a pleasant relief following the massive revisions brought about by the 1986 rewrite of the tax law.

Among the modifications this year:

- The personal exemption, the amount you may exempt from taxation for yourself, your spouse and each dependent, increases to $2,050, up from $2,000 last year.

- The standard deduction, claimed by people who do not itemize, rises to $3,250 for a single person, up from $3,100 last year. The exemption for a couple increases to $5,450, up from $5,200 last year.

- Families with income as high as $20,264 may qualify for an earned-income credit, up from a limit of $19,340 last year. The maximum credit available is $953.

- The basic tax rates remain the same at 15 percent, 28 percent and 33 percent, but the tax brackets have been widened so that more income will be taxed at a lower rate, thus compensating for the effects of inflation.

- Only 10 percent of interest paid on credit card debt, auto loans and other personal debt will be deductible on 1990 returns, down from 20 percent last year.

The IRS said it expects 113 million returns will be filed this year and that 70 percent of all taxpayers will get refunds. The average refund last year was $900.

Two-thirds of all taxpayers are expected to file the long Form 1040, which is used by persons who itemize deductions. The other one-third will use either the simpler Form 1040A or the nine-line Form 1040EZ.

In an effort to encourage use of the shorter forms, which are easier to process, the IRS made an important change to the Form 1040A this year which will allow 4.5 million elderly persons to use it to report retirement income they previously could report only on the longer 1040.

These taxpayers will receive the shorter form with an explanation of the change.

The IRS is again encouraging people eager for their refund checks to use an electronic filing service. The fee normally ranges from $25 to $50 but it usually means a refund check will be mailed out in two weeks to three weeks, versus the five weeks to six weeks it takes to process a refund from a paper return.

People who need tax forms other than those included in their package can call a new IRS toll-free telephone number, 1-800-TAX-FORM (1-800-829-3676).