This is the season of the tax collector, and Monday, April 16, is a key day for taxpayers-the day by which most people must file their federal individual income tax returns.

There is an exception: If you are not in the United States (or Puerto Rico) on April 16, you are allowed an extension to June 15, without having to file a request. When you do file, you must attach to your return a statement that you were outside the country on April 16; in addition, you are required to pay interest (from April 16 to the date of payment) on any unpaid balance.

Taxpayers who are not out of the country may also get an extension to June 15, but it must be requested. Aproval is automatic-in fact you won't even get an acknowledgement; but you must file IRS From 4868 by April 16 to qualify.

Form 4868 is easy to complete; but you must estimate what you expect your tax liability will be for the year. You should send payment with the form for any expected deficiency; but you can't request a refund for an expected overpayment-that must wait until you file the completed return.

File Form 4868 with the Internal Revenue Service Center where you normally file your return: Philadelphia, Pa. 19255 for Maryland and District residents, or Memphis, Tenn. 37501 for Virginians.

April 16 is also the due date for District and Maryland returns. Residents of D.C. can get an automatic two-month extension simply by filing a copy of federal Form 4868 with the Department of Finance and Revenue by April 16. (Attach a second copy to your D.C. tax return when iis filed.)

For a Maryland extension, however, you must submit a letter to the Comptroller explaining the reason for the request, together with a copy of federal Form 4868.

Virginia state tax returns are not due until May 1. If you need an extension beyond that date, file Virginia Form 760-E with the Department of Taxation by May 1. This will automatically get you an extension to a date 15 days after the expiration of the federal two-month extension-that is, to June 30 for most taxpayers.

If you're required to file an estimated tax return for 1979, the due date for the estimate (and the first quarterly payment) is also April 16 for the federal government, the District of Columbia, and Maryland. In Virginia the date for filing an estimated return-May 1-is the same as for the annual tax return.

In all jurisdictions, the succeeding quarterly payments are due June 15 and September 15 in 1979 and January 15, 1980. But if you want to be able to claim the fourth quarter state payment as an itemized deduction on your 1979 federal return, send off that payment by December 31, 1979.

If you wait until January 15 of next year, you still take credit for the payment against your 1979 state tax liability; but you have to wait until your file your federal return for 1980-the year of payment-to deduct a January payment on federal Schedule A.

April 16 is also the final day to take advantage for the tax-saving feature of a Keogh or IRA retirement plan. A Keogh program-a retirement plan for the self-employed-must have been established by Dec. 31, 1978; but deposits on behalf of 1978 earnings may be made as late as Arpil 16 and still qualify for preferential tax treatment on your 1978 return.

An Individual Retirement Arrangement (IRA) is designed for employees who are not covered by a pension or retirement plan where they work. (The IRA may also be used by a self-employed individual in place of a Keogh plan, if the lower ceiling on contributions is not an important factor.)

The deadline for IRA deposits based on 1978 earnings is also April 16. Unlike a Keogh plan, however, the program need not have been previously established. If you are eligible, you can open an IRA account on Monday, April 16, and still deduct the amount of a qualifying deposit on your 1978 tax return.