If you have received a job promise from a federal agency within the last 20 days, don't put your house on the market or your goods in a moving van just yet in order to move here. Unless the 'you're-hired" telegram or letter was postmarked before Dec. 31, you can probably forget about coming to work for Uncle Sam in the near future.

Insiders say the tough, no-hiring freeze President Reagan will impose on the bureaucracy this week contains language to the effect that no job promises made after Dec. 31 are to be honored. Federal personnel directors say they have been told by Reagan transition officials that the freeze is definitely coming, and that the new administration doesn't want it fuzzed up by having to honor hundreds of thousands of job promises made in the closing days of the Carter administration.

Agencies that promised people jobs in recent weeks, after it became clear that Reagan meant what he said about a freeze, may now have to start writing letters to people unpromising them jobs.

In normal times, Uncle Sam hires about 1,500 people every day to replace workers who quit, die or retire. That inflow has been cut since last March by a Carter directive telling agencies to fill only one half of their vacancies. The 1-for-2 partial hiring freeze resulted in a decline of between 20,000 and 23,000 permanent, full-time federal jobs. But in recent months -- and especially in the last few weeks -- some agencies have been "overhiring" by taking people on board in anticipation of coming retirements and other vacancies. Some have been moving quickly to fill slots that were exempted from the partial hiring freeze because of the nature of the job.

Whatever the reason for the recent hiring surge, the Reagan people are ticked off about it. They think that agencies and departments -- one in particular is the Department of Education -- have been working overtime to get "commitment letters" to people to beat the coming freeze.

President Carter "telegraphed" his intention to slap a freeze on federal hiring early last year. Freeze rumors hit the press and spread through the bureaucratic grapevine. Thousands of commitment letters were sent out to beat the deadline. When Carter did impose the freeze he let agencies honor commitment letters they had sent out. But agencies had to make it up later on out of their 1-for-2 hiring authority.

Reagan brass say his freeze, which is expected to be relatively brief but solid, is not designed to let agencies take hundreds of people on board after the freeze is ordered today or Wednesday.

So if your commitment letter isn't postmarked on or before Dec. 31, you will want to check the private sector want ads until the dust settles.