Most of the nation's retirees, about one in every seven people, are tentatively due a minimum 3.7 percent January raise based on this year's cost-of-living countdown.

If the raise holds up, it will mean that retirees will get bigger increases than active duty federal and military personnel, who are expected to get increases of between 2 and 3 percent. Raises for active federal personnel are decided by Congress and the White House, whereas retiree increases are linked to living costs.

The automatic retiree raises will go to Social Security recipients as well as federal-military retirees. There are about 100,000 U.S. retirees here and many more people -- some of them who are also federal or military retirees -- who will get the Social Security raise.

By law, the retirees are supposed to get January raises to help keep pace with inflation. Each increase is based on the rise in the consumer price index from the third quarter (July, August, September) of the previous year.

The Labor Department's consumer price index for July shows living costs up 3.7 percent over the third quarter of 1986. If the index goes higher, as expected, retirees will get an even bigger raise. The typical federal retiree here gets $13,566; the average survivor benefit is $6,432 per year. Stamp Story Unglued

Postmaster General Preston Tisch says forecasts of a 30-cent stamp within three years are "pure fantasy." The Postal Service has asked that the 22-cent price of a first-class stamp be increased next year to 25 cents. Last week this column quoted the Business Mailers Review newsletter as saying that the new multibillion-dollar contract between the service and its 800,000 workers could drive the stamp price to 30 cents in three years. That contract, biggest in the nation this year, calls for 13 raises over 40 months. But postal officials say its costs were taken into account when they asked for the 25-cent rate.

Copies of the newsletter, which said that the Postal Service had caved in to union demands, were included in informational packets one of the unions sent members along with material urging them to ratify it. But Tisch said the contract costs will run "below those projected for the private sector and they will not increase our need for revenue in the present rate case." Aides say that means that if first-class stamp prices go to 25 cents next year, that rate will be in effect at least through 1991.

Meantime, members of the National Association of Letter Carriers union have voted overwhelmingly to accept the new contract. The mail ballot ratification vote was 112,795 to 16,476. About half the union's members voted. Members of the American Postal Workers Union also are expected to approve the contract. The contract promises workers six regular raises during its lifetime, plus seven cost-of-living adjustments. It will push the pay of the average clerk and letter carrier, now $27,401 per year, well past the average $27,747 salary for white-collar federal workers. Nonpostal employes are due a raise in January, but get no cost-of-living protection as do their highly unionized postal colleagues. Day Care

A special learning center is to open at the Interior Department next month. The 7 a.m. to 7 p.m. service is for the children of employes of Interior, General Services Administration and Office of Personnel Management. Fees range from $87.50 to $115 per month for children from 3 months to 5 years old.