Federal workers anxious to retire before Congress makes any tax change affecting their pensions probably won't have to make a decision until at least December.

Although the House has proposed that a change go into effect in July, congressional insiders expect the actual effective date -- if a change is made at all -- will be Jan. 1 or later.

An estimated 200,000 government employes, including about 40,000 here, could retire now. Many have indicated that they will leave if it appears that Congress is going to change the tax status of their pensions.

There are two good reasons to speculate that the tax change won't come in July. First, Congress is still a long, long way from approving any kind of tax reform bill. It is highly unlikely both that it will be approved by midsummer and that it would later be made retroactive.

Secondly, many members of Congress and key congressional staffers plan to retire in early January. Most would prefer to retire before any pension tax rule change goes into effect.

The benefit that Congress is considering taking away is the so-called recovery rule. It exempts from federal taxes the annuities of those who contribute to pension plans until they retire and recover all of the already taxed money they paid into the plans. That recovery period for civil servants typically is about 18 months.

A spokesman for the Senior Executives Association, one of the leading opponents of the pension tax change, said yesterday that it is SEA's understanding that there is an unwritten agreement between the Senate Finance Committee and the House Ways and Means Committee to make the change effective in 1987 rather than in July.

Meanwhile, Sen. John H. Chafee (R-R.I.) has pledged to oppose the pension tax change when it comes up for a vote in the Senate Rules Committee.

An aide said that Chafee feels it would be "unfair to change the rules now" on federal workers, state employes and others "who have based their retirement planning on current tax rules."

Several Democratic committee members are expected to side with Chafee. If they are successful, the Senate tax reform bill would not make any change in the recovery rule. Senate-House conferees would decide later whether to drop the controversial measure from the package that will go to Congress. Meetings

D.C. Del. Walter Fauntroy will speak at the April 9 meeting of the Chevy Chase chapter of the National Association of Retired Federal Employees. The meeting is at 12:30 p.m. at the Chevy Chase Community Center. For details call Bob Rosenthal, 966-8127.

The Washington Society of Engineers meets at 8 p.m. Wednesday at the Cosmos Club. James Lounsbury of the Environmental Protection Agency is the speaker. Call Richard Brown, 836-7528.

Labor Department's David F. Demarest is the luncheon speaker at today's meeting of the International Association of Personnel in Employment Security. It will be held in the multipurpose room of the Patrick Henry Building, 601 D St. NW. For details call Geraldine Wulff, 639-1731. Job Mart

Bureau of Engraving and Printing has openings for a secretary (typing), Grade 6, and a GS 12 computer specialist. Call 447-0544.

Army's Harry Diamond Lab has openings in Adelphi and McLean for full-time and part-time clerk-typists, GS 3/4, and secretaries (typing and steno), GS 4/5. Call 394-2816.

Agricultural Stabilization and Conservation Service needs a GS 11/12 computer systems programmer. Must have civil service status. Call 447-7518.