Y2K may be less than 100 hours away, but it's hard to discern that at the McLean headquarters of Freddie Mac, the gigantic housing finance corporation that relies on computers for almost every part of its business.

The computer control center, a darkened room with large, wall-mounted screens and rows of terminals, is staffed with only a handful of technicians--the contingent for a normal business day. The company's Y2K "response center" is empty. Some programmers are working on projects that have nothing to do with the date glitch and a few others are on vacation.

"It's just like any other day," said Bill Ledman, Freddie Mac's chief information officer. "We're just waiting for the weekend to get here."

This weekend is when things will get frenetic, with programmers, systems analysts and executives pulling round-the-clock shifts to test their computers and repair any previously undiscovered Y2K glitches. But for now, unlike a political campaign headquarters on election eve or Cape Canaveral before liftoff, the vast majority of the country's computer rooms haven't been a beehive of activity.

Most of the crucial repairs were completed months ago, and final testing and contingency planning, in most cases, wrapped up weeks ago. "It's the calm before the storm," said James Woodward, a senior vice president at Cap Gemini America Inc., an information technology firm that has performed Y2K repairs for large businesses. "At this stage, there's not much to do. It's a little like waiting for Santa to arrive."

The tranquil nature of most of the country's computer centers is one of the strongest indications of the confidence that American businesses and government agencies have in the Herculean efforts they have undertaken over the past several years--at a collective cost of more than $100 billion--to inoculate themselves against the "millennium bug."

"People are feeling good about the work they've done," Woodward said.

The pace already has quickened at other locations. At businesses expecting a surge in consumer activity in the days leading up to Jan. 1--banks, grocery stores and gas stations--executives and computer specialists already have been pulling longer hours.

BankBoston, for example, has stepped up 24-hour monitoring of its computer systems, particularly watching the demand for cash at its automated teller machines, said David Iacino, the director of the bank's Y2K project.

"This is a high-volume period for us, and we want to have our finger on the pulse of everything that's going on," he said.

Making any changes to highly complex corporate computer systems--many of which interconnect with suppliers, customers and other business partners--often can introduce new glitches, potentially causing more harm than good, according to technology specialists. As a consequence, programmers wanted to finish repairs well before the end of the year to give themselves plenty of time to test and re-test their fixes.

Sanguine that they have tackled the date glitch, most businesses now are in a "lock-down" mode, which means no new software can be installed, even if it is unrelated to Y2K. "Companies have verified that their systems work and they don't want to introduce any new complexity into the mix," said Matt Hotle, a vice president at the Gartner Group, an information technology consulting firm.

Whether or not those systems operate as planned come Saturday is still an open question. "Right now, there's a great degree of fatalism," Hotle said. "What's going to happen is going to happen. There's little they can do about it now."

At Bell Atlantic Corp., fixes to "mission critical" computer systems were made by the self-imposed deadline of June 30. That meant the spring was particularly hectic for the phone company's programmers. Late nights. Weekend shifts. No vacations.

"That's when there was blood on the floor," said spokesman Jim Smith.

Nowadays, "people are quite bored because there's nothing to do," said Skip Patterson, Bell Atlantic's Y2K director. "We've done all the things we can think of to do and now we're like the hosts waiting for the guests to show up for the party."

Nevertheless, Patterson said Bell Atlantic doesn't plan to let its guard down. Those employees deemed critical to the Y2K effort haven't been allowed to take extended vacations, and more computer technicians are on duty than usual.

Like many businesses, Bell Atlantic has found some problems over the past several weeks in systems thought to have been fully repaired. About 2,000 monthly bills, for example, were printed with the year 1987 instead of 2000, a goof that was quickly corrected, Patterson said.

Come Friday afternoon, though, the placid workplace will give way to a full-scale mobilization at Bell Atlantic and most other firms. The phone company plans to have 20,000 employees on duty over the weekend, 30 percent more than normal, Smith said.

At BankBoston, although "nobody is walking around chewing their nails," Iacino said the anxiety level among his programmers is increasing slightly as New Year's Eve draws closer. "Nobody knows for sure what's going to happen," he said. "The only certainty is the uncertainty."