Tax Calendar: If you make quarterly payments of estimated tax for 1978 -- federal, D.C., Maryland or Virginia -- the fourth and final payment is due Jan. 15.

Question: We installed carpeting of top of the tiled floor in our family room to keep the floor warmer and save on heating costs. The tiles had been laid directly on a concrete slab and always felt cold. Does this qualify for the energy credit ?

Answer: No. The rules governing the tax credit for energy conservation are quite specific in naming the measures that qualify, such as storm doors and windows and insulation.

In addition, you can claim the credit for installing an insulating jacket around a water heater, an energy-saving thermostat with automatic setback, or an electronic ignition system or heat-saver flue on a furnace.

Specifically excluded are such things as a Franklin stove or wood-burning furnace, exterior siding, and -- sorry about that -- drapes or carpeting.

@q: i/ own a four-unit apartment house that I now plan to sell. I expect that there will be a substantial capital gain. Is there anything I can do to reduce the tax I have to pay ?

A: One thing already has been done for you. The Revenue Act of 1978 gives you a break on the amount of tax that will be due. Assuming you have owned the house for more than 12 months, you will have to report only 40 percent of the gain with your other income instead of the 50 percent previously reportable.

Modifications for 1979 to the minimum tax structure governing tax-preference income (including the capital gains exclusion) also may work in your favor, particularly if the excluded portion of the gain doesn't exceed $60,000.

If your taxable income (including only the taxable portion of the capital gain itself) is substantially higher than in previous years, you may be able to reduce the tax bite by income averaging on Schedule G. (However, there is an interaction between income averaging and the new alternative minimum tax, so check this one out carefully.)

Another option is to arrange for payment on the installment plan. If you receive no more than 30 percent of the sale price in the year of sale, with additional payments in at least one other tax year, you only report each year the proportionate gain on the payments actually received in that year.

This method keeps you in a lower tax bracket by spreading the gain over several years, thus reducing the total amount of tax paid. (Of course, you also must report as income any interest received on the payments.)

Q: We have been purchasing U.S. Series E bonds for our son's education. The bonds were isued in his name, with his mother as beneficiary in the event of his death. We would like to cash in these bonds and invest the proceeds in something offering a higher return -- but our bank says they can't be redeemed until our son (now 12 years old) reaches 18. Ture ?

A: Not true.The bonds can be redeemed at any time after you have held them for two months.

The confusion probably arises because some investments -- corporate stocks, for example -- cannot be owned by a minor. They must instead be held by a "custodian for a minor child," usually a parent.

But the Treasury Department specifically has authorized purchase, ownership and redemption of Series E and Series H bonds by minors. If the bank asked to redeem the bonds is satisfied that your son has reached the "age of reason" -- normally about 12 -- it can accept the child's signature alone. Otherwise, it may request the concurrence of a parent or guardian.

In any case, the bonds certainly can be redeemed now, Try another financial institution; you should have no problem cashing in the bonds to invest the proceeds elsewhere.