(CNN) -- The rush to book airline tickets for millennium travel officially begins Thursday, the first day that most airlines will accept bookings for departures on January 1, 2000. The day will be a test for the big computer reservation systems that handle the majority of flights.

Although travel agents have been booking packages for New Year's Eve 2000 for some time, flight booking systems aren't able to work until 330 to 331 days in advance, meaning that February 4 is the first day that passengers can book tickets on specific flights departing on January 1, 2000.

After working for a half decade and spending billions of dollars, the travel industry will discover if its computer reservation systems are able to handle the year 2000 rollover -- or if the computers will mistake January 1, 2000 for January 1, 1900.

More than 100,000 travel agents that book about 80 percent of all flights depend on reservation systems operated by only a handful of computer reservation system companies.

If those companies and their partners are able to handle the year 2000 rollover on Thursday, the travel industry will have shown it is well on its way to crushing the once-dreaded Y2K bug -- which stems from the once-common practice of using only two digits for recording the year in computer programs, like "99" as an abbreviation for 1999. The shortcut has the potential to confuse computers and software and cause them to produce bad data, or not work at all.

Ronnie Hauptman, Year 2000 director for Galileo International, plans to be up all night on Thursday watching for rogue data flashing across her screens at the company's Denver operation.

Galileo is the world's second-largest computer reservation system company. It handles about a third of world travel bookings. Sabre Group Holdings, the No. 1 travel reservation company, handles another third. Another major player is Spain's Amadeus Global Travel Distribution, in which Iberia, Air France, Lufthansa and Continental Airlines own stakes. WorldSpan, in which Northwest, TWA and Delta have stakes, is the fourth largest system.

Analysts are watching to see if the reservation system companies can protect their networks from data that is not ready for the year 2000 and handle internally any problems that could arise.

Meanwhile, airlines are preparing to handle an expected crush of bookings. An estimated 639 million travelers are expected to take to the skies in the year 2000, more than ever before in aviation history.

New Year's Day and the days immediately following are traditionally among the busiest of the year, so finding flights for a demanding public will be challenging. Most major airlines are trying to prevent flyers from booking multiple tickets by placing special restrictions on high-demand bookings during the millennium season.

	Would you want to travel on January 1, 2000?
	Yes. It only happens once
	No. I don't think it's safe
	I might consider it
	Who cares? It's all hype anyway
	View Results

The specific rules are different for each airline. United, Delta, US Airways and Northwest are restricting tickets to Florida, Arizona, Las Vegas, New York City and New Orleans.

"The travel industry in general will put restrictions on tickets, on hotel reservations, on car rental reservations during peak periods... Tickets that are normally fully refundable, during this period are going to be non-refundable, so once you've made your reservation, you've got to stick to it," said Chris McGinnis, of Travel Skills Group.

Some airlines have taken Year 2000 bookings since January 3, and cruise lines have been taking them since last year. All of that has passed without incident, analysts said.

The airlines say they are ready for the onslaught of millennium ticket buyers.

"A number of airlines have already started ticket sales and everything has gone smoothly," said David Fuscus, a spokesman for the Air Transport Association. "(W)e don't anticipate any problems at all, because you have to remember, every day is a day when we start selling tickets for some day 330 days in the future, so this is very routine for us."

However, the travel industry will still face some key tests after Thursday. Although airlines and car rental companies begin taking bookings on Thursday, hotels start handling reservations for the year 2000 on Friday.

In addition, there is September 9, 1999 -- a day that creates a row of four nines, which could crash some computers or fool them into deleting data.

But if the industry can get past Thursday, it can probably jump any hurdle, analysts said.

"It really just is an acid test of how successful they've been in year 2000 compliance -- whether or not they've made the right investments," said Dillon Warburg's Michael Stellwag.

CNN Correspondent Stephanie Oswald and Reuters Limited contributed to this report.