Every Dec. 31, Gene Troxell, his wife and several friends celebrate the new year with a small party in their Loudoun County neighborhood. It's become a tradition of sorts, one that he expects to be particularly memorable this winter as everyone rings in the new millennium with champagne and a gourmet dinner.

Everyone, that is, except him.

Troxell, Loudoun's director of information technology services, will be at work all night. Camped out in a sterile data center, he will monitor the county's 40 mainframe and "file server" computers -- which process everything from requests for police assistance to property tax bills -- for year 2000 glitches. If any occur, Troxell needs to be on the spot, ready to assess the situation and direct emergency repair efforts.

Although he'll have the company of several fellow computer experts that night, there won't be any bubbly or fancy repast. Everyone has to stay sober -- just in case. And there isn't a place in the data center to prepare or serve an elaborate dinner. Grouses Troxell, "I'd much prefer to be spending the night somewhere else."

Troxell's lament is becoming an increasingly common Y2K problem. Across the globe, hundreds of thousands of workers recently have found out -- or will soon be notified -- that their New Year's Eve party and vacation plans have been squashed by the millennium bug.

In many cases, it's not just techies who are affected. Most businesses and government agencies have drawn up elaborate "contingency plans" that require many of their nontechnical employees to be on duty or on call from late December to early January in case tasks that now are automated need to be processed by hand.

Washington Gas Light Co., for instance, has told all of its nearly 2,000 employees -- from field technicians to customer service representatives -- that they will not be permitted to take vacations between Dec. 27 and Jan. 7. Potomac Electric Power Co. has delivered a similar edict, banning leave from Dec. 26 to Jan 8. for its 3,700 workers. And Inova Health System, which operates five hospitals in Northern Virginia, has notified 300 employees who work on computer systems and electronic equipment used for patient care that they need to be sober and within an hour's drive of their hospital on Dec. 31 and Jan. 1.

"Employees are a key component of contingency planning," said Washington Gas spokesman Tim Sargeant. "Having them on call will enable us to handle whatever scenario may come up."

The year 2000 problem, commonly known as Y2K, stems from the fact that millions of computers were programmed to recognize only the last two digits of a year, assuming the first two would be 1 and 9. When Jan. 1, 2000, arrives, unprepared machines will understand the year "00" not as 2000 but as 1900, potentially causing them to shut down or stop working properly.

Although businesses and government agencies have spent billions of dollars in a massive effort to repair and test their systems, no one knows exactly what will happen in the first few hours and days of the new millennium. As a result, almost every organization plans to have its computer specialists and other key personnel in or around the office over the holiday weekend.

A recent poll of 200 technology executives conducted by Chief Information Officer magazine found that more than half plan to ban vacations for their computer technicians at the end of the year. And a vast majority of executives surveyed anecdotally by industry analysts say they intend, at the very least, to require that their tech staff be near the office and refrain from drinking.

"We're telling them to demonstrate the normal good judgment they would before coming to work," AT&T Corp. spokesman Dave Johnson said of the "several thousand" employees at the telecommunications giant who will be on call over the holiday weekend.

AT&T, like many large businesses that provide crucial, round-the-clock services, will have a core group of technicians who will monitor the date changeover from a "command center." Standing by will be several "service resumption SWAT teams," which will be able to install new equipment or cables if needed, Johnson said.

At some organizations, technicians won't be baby-sitting computers at midnight but will be trekking to the office early on Jan. 1 to run tests to ensure everything is working properly. The Social Security Administration, for instance, plans to station only 50 employees and managers at its Baltimore data center overnight, but at 8 a.m. on New Year's Day, 400 computer specialists have been asked to come in to analyze the output of various systems. And across the country, supervisors will conduct inspections at the agency's 1,350 field offices, checking to see whether computers, elevators, lights and heating systems are working.

Around the Washington region, local governments are determining which employees will be denied vacations. In Fairfax County, all 1,110 police officers have been told they cannot request leave between Dec. 27 and Jan. 8. The officers and police administrative staff have been informed they may have to work 12-hour shifts on those days, a schedule that could, if needed, double the police presence on county streets.

Similar leave restrictions for police officers have been established in Montgomery County and the District.

The Metropolitan Washington Airports Authority, which operates Dulles International and Reagan National airports, has told its 1,100 employees that any vacation requests between Dec. 27 and Jan. 5 must get special approval. "We don't know how many people we eventually will need, but we don't want to be in a position where the people we need are out of town," spokeswoman Tara Hamilton said.

At a handful of businesses, the vacation ban doesn't just cover the New Year's holiday. With concern about computer glitches on Feb. 29, the first "leap day" in the first year of a new century since 1600, Washington Gas employees also will be prevented from taking days off between Feb. 22 and March 7.

The federal government is limiting vacations on an agency-by-agency basis. Some, such as the Federal Aviation Administration, plan to require a sizable portion of their work force to be on duty or on call. And to discourage Y2K workers from asking for time off between now and the end of the year, the Office of Personnel Management recently said it would allow such employees to roll over any excess vacation time until next year instead of requiring them to use it before Dec. 31.

Several countries have taken the unusual step of canceling leave for senior military officers at year's end, in part to have them available in case of any glitch, but also to encourage them to proceed more quickly with Y2K repairs, said a Clinton administration official, who did not identify the countries for security reasons.

The prevalence of vacation restrictions, coupled with a general uncertainty about the effects of the Y2K problem, has caused a drop-off in people's plans to leave town for the end-of-year holiday, say travel specialists. Although it once was thought to be nearly impossible to get an airline ticket to such destinations as New York and London -- or a hotel room -- without a years-in-advance booking, such reservations can be made easily, if expensively, today.

"People who were really excited about going on a millennium vacation a year ago are now changing their minds and deciding to stick close to home and celebrate here," said Kim Peele, who runs a travel agency in Alexandria.

Mindful of the sacrifices their employees are making on the biggest party night of the century, many organizations are planning small -- and muted -- celebrations at the office. At National and Dulles airports, for example, there will be a buffet dinner and toasts with nonalcoholic sparkling apple cider. At the Inova hospitals, workers will be treated to a "post-Y2K" party and will get to take off the first Friday in January, spokeswoman Lisa Wolfington said.

Industry and government executives say their no-vacation policies haven't generated any widespread disenchantment among employees. In most cases, there is the emolument of overtime pay or the ability to take time off after everything's fixed.

"This doesn't come around very often," said AT&T's Johnson. "It's a once-in-a-lifetime opportunity to be a part of something very unique. Ten years later, they can say, `I worked the night of Y2K.' "

CAPTION: As Loudoun County's director of information technology services, Gene Troxell won't be off New Year's Eve.