This is a very bad day to call in sick. Or have car trouble that will keep you out of the office all day. This is that most suspicious time, the Monday before the biggest holiday of the summer.

According to the calendar, this is just another work day for the metro Washington area, which regularly receives a $700 million monthly infusion of government dollars in the form of U.S. and military paychecks. The buses will run (or are supposed to) on regular schedules. The subway will run, or stall, as usual for a Monday. But if you are one of those people left in town, you may notice things are slightly different at the office.

Federal officials estimate that anywhere from 30 percent to 60 percent of the people who actually run the government machine will be off today. Either at the beach, mountains or sitting home fiddling with the fan or air-conditioner.

Generally government agencies are taking a "liberal annual leave" policy, meaning that almost anybody with vacation time is permitted to take it either as a miniholiday today, or to get a long Saturday to Sunday nine-day holiday using only four days of annual leave.

Because this is a workday, many area businesses will maintain normal schedules, and bus routes will be run as per usual, and federal cafeterias are planning regular menus. For those left behind to run the government store, this should be a relatively quiet, peaceful day.

Job Insurance; The House Post Office-Civil Service Committee has added a job-demotion-protection rider to the President's civil service reform package.

Under the bill, by chairman Robert N.C. Nix (D-Pa.), federal agencies would be prohibited from demoting or downgrading workers because of reorganization, or because desk audits determined that the job had been erroneously overgraded. The bill would not protect employes from demotions resultin from reductions -in-force, or for cause.

The Nix bill, now part of the reform package on the House side, also would restore workers hit by "no fault" demotions since January 1977 to their former grade. President Carter has endorsed the legislation, but Administration officials don't like the fact that his reform bill is being used as the vehicle to carry other pieces of legislation through Congress.

House leaders decided that the only way the no-fault demotion bill will get through Congress this year to have it tied to a much stronger piece of legislation with the full backing of the White House.

The Senate Government Affairs Committee already has finished work on the reform package, but Senate leaders are expected to okay the demotion-protection bill when the House version reaches them.

Dead Letter? The July issue of the Readers Digest has a thought-provoking piece, by James Nathan Miller, about the current status of the U.S. Postal Service.

Miller believes the postal service is pricing itself out of business, losing some of its best customers to private industry. Among them, perhaps, is the Readers Digest.

The magazine, one of the world's biggest mailers, already is experimenting in a big way with nonpostal delivery. In a California test, a private firm now is delivering copies of the Readers Digest to 150,000 subscribers. Magazine officials say the price is the same as the present fee charged by the postal service, and that private delivery will be 2.5 cents per copy cheaper next year when new magazine rates go into effect.

The Digest article will anger some postal officials and some union leaders. But they ought to read it anyhow, because a lot of others will be reading it too.