Airline and airport personnel are gearing up for the heaviest concentrated travel rush of the year -- government workers dashing home from Washington for an extended Thanksgiving weekend. (However, the influx of people, notably college students, coming home to Washington for family reunions is considerably less than the outflow.)

Time was when Wednesday, Thanksgiving Eve, produced the biggest one-day exodus of the year, but airlines serving National Airport -- and to a lesser extent, Dulles, which generally serves longer-haul passengers -- report that travelers are now stretching out their holidays by leaving earlier in the week.

As a result, one spokesman said, Tuesday and Wednesday should be about even in total passengers using the airports, but there still will be one heckuva snarl on National Aiport's access roads after the government shuts down operations Wednesday afternoon.

David Hess of the Federal Aviation Administration, which runs the airports, urged passengers to travel if at all possible to National by subway or, lacking that, to allow an extra half hour or so to reach there by car or cab.

"Sunday," when almost everybody wants to return, said Hess, "will be a pipparoo."

For one thing, those arriving at National after 6 p.m. Sunday will find our Metro subway routinely closed, with the scheduled buses headed downtown few, far between and extortionately expensive ($1.30 instead of 65 cents on the train).