Most of the nation's federal workers -- including more than 300,000 here -- will get an early start for Christmas, thanks to an executive order issued by the president giving them a half day off today.

The four hours of paid leave can be granted at the discretion of agency heads. The bonus holiday doesn't apply to the U.S. Postal Service or to people in key jobs in national defense, security, air traffic control, hospitals or other essential services that must be staffed round-the-clock.

Washington area merchants may get added business from the surge of last-minute Christmas shoppers, and Metro traffic planners must decide whether to step-up rush hour bus and subway schedules to accommodate thousands of people who will be leaving early.

Because the half day off is considered a holiday, many employes who are required to work will be paid overtime for the half day. The last time Christmas fell on a Friday was in 1981 and at that time the president also gave workers a half day off.Executive Pay Raises

Despite the congressional freeze on pay raises for high-paid federal workers, some members of the Senior Executive Service -- those in Executive Service Levels One, Two and Three -- may get the regular 2 percent raise that goes into effect next month for rank-and-file white-collar federal workers.

SES members in Levels Four through Six will not get the raise because the congressional pay freeze covers anyone making $72,500 or more per year.

If the president issues an executive order raising SES pay, these will be the new rates: ES Level 1, $65,994; ES Level 2, $68,952; and ES Level 3, $71,910. Pay rates for employes in higher levels will not change.

The SES raises will not go into effect unless the president specifically orders them. Retiree Raises

Retired federal workers, military retirees, their survivors and Social Security recipients will be getting a 4.2 percent cost-of-living adjustment in their January checks. The COLA payment -- which reflects the rise in living costs this year -- was approved as part of the White House-congressional budget package that was signed into law this week. Budget Rundown

As previously reported here, the budget compromise and stopgap funding bill signed by the president make several important changes in federal retirement rules, including lump-sum pension benefits and eligibility for unearned Social Security benefits.

Because the changes were made at the last minute by Congress, most federal agency personnel and retirement offices still don't have the details on what those changes are or how they worked. Tuesday's column carried details of the Social Security and lump-sum payment changes, and yesterday's column outlined improvements Congress made in the federal employes thrift investment plan. On Sunday we will have a complete rundown here of all the changes, including new retirement deadlines for some employes.