TOKYO -- For young people all over the Christian world, this evening will be a night of magic and wonder. Here in Japan as well, Christmas Eve has become immensely important -- but for rather different reasons.

It has become the sexiest night of the year.

Japanese popular culture has made Christmas Eve a night when every unmarried person must have a date, and it is now de rigueur for the date to include an overnight stay. For weeks now TV shows, magazines and manga (adult comic books) have been full of reports and advice on which hotels are best for young couples to stay in on Christmas Eve, what each partner should wear, and where the pair should have breakfast the next morning.

Virtually every major hotel in Tokyo reports that Dec. 24 has been sold out for months. At the popular Sheraton Grande Hotel, which has installed a larger-than-life plastic Nativity scene in its lobby to add to the ambiance, all rooms have been reserved -- and paid in advance -- since April.

"Christmas Eve is now important as a night for making love," complains poet and social critic Hazuki Kajiwara. This is such a widely accepted aspect of the day known here as eebu, the Japanese pronunciation of "eve," that Dec. 24 is frequently referred to as "H-Day." The letter H, taken from the English word "hormone," is a common symbol here for sex.

The eebu phenomenon is carried out in an intensely materialistic, free-spending atmosphere, reflecting the commercial nature of the Christmas season in Japan. In a country less than 1 percent Christian, Dec. 25 is just another working day; yet stores and restaurants here have more Christmas trees, wreaths and reindeer on display than most places in the United States.

For a couple's Christmas Eve fling, the man is expected to bear all costs. Many "salary men" save all year for this one date. Media reports here estimate that the cost of a typical couple's Christmas Eve will exceed $1,000.

The newsmagazine Asahi Journal last week printed a breakdown of a fairly standard "Eve course" -- that is, the itinerary of a Tokyo couple's Christmas Eve date. The itinerary, incidentally, gives the lie to the notion that Japanese people won't spend their money on American goods.

When the man arrives to pick up his date, the magazine said, he should present her with a $215 silver heart pendant from Tiffany and then take her out for an evening at Tokyo Disneyland, where admission and extras will cost $100 or so. Then it's on to dinner for two at a French or American restaurant ($385) and a room for the night overlooking Tokyo Bay at the Hilton or the Sheraton Grande ($300, or $650 for a suite). Breakfast in the hotel coffee shop should cost only $35, but a rental limousine to take the couple to their homes so they can quickly change and go to work will cost another $150.

Things would be cheaper if the couple could just go to one of the thousands of "love hotels" here, where a room costs about $30 for two hours. But the popular magazines have decreed that this is too tacky for such a special night.

But eebu is hardly a free ride for Japanese women. They must pay the emotional cost.

Women between college age and their mid-thirties have more money and more independence today than ever before in Japanese history. But they are losing connection to family and peer groups, and are struggling to survive on their own.

"Behind the traditional Japanese groupism is a fear of being alone," says Hikaru Hayashi, senior research director at the Hakuhodo Institute, a sociological think tank. For single women, "Christmas Eve enhances the fear that they are not rooted to society."

With everybody making elaborate plans for eebu, it is a social necessity for single women to have a date that night. The tribulations of those who don't have become the subject of enormous media attention.

A travel agency called Kinki Nihon Tourist (the word "Kinki" here is strictly geographic, referring to the central section of Japan's main island, Honshu) has been advertising excursion trips for singles this fall under the headline "Find a Boyfriend by Christmas!"

This fall, the Tokyo Broadcasting System ran a 12-part miniseries called "Christmas Eve." The story concerned a young "office lady" who listened to her friends chattering about the fancy restaurants and hotels they were going to for eebu but was ashamed to admit she had no date. Last week, in the final episode, a young man called her at the last minute. The two walked off happily into the night, presumably in search of a hotel with a vacant room.

The popular comic book serial "Office Lady Story" this month features an episode called "29-Year-Old's Christmas." It features an office worker who comes home to her one-room apartment to find a piece of junk mail advertising a Christmas party where single women can meet men. When she sees the price, $400, she angrily throws the flier away. Later that night, though, sitting alone with her hair in curlers, she pulls it out of the trash and says, "Well, it's only $400... . "

The idea that it might be shameful for a single woman to spend the night with her date is less commonly expressed, but it does occur. Sampei Sato, the widely followed editorial cartoonist for the national newspaper Asahi Shimbun, devoted his space one day this month to an appeal to young unmarried women to sleep at home rather than in a hotel on Christmas Eve.

While young men are emptying their wallets and young women are waiting for the phone to ring, the rest of Japanese society, never shy about analyzing itself, is busily talking about how Christmas Eve became a time for sex.

One theory involves a pop singer named Yumi Matsutoya, who had a hit record titled "My Boyfriend Is My Santa Claus." In a homogeneous society where people swarm to each new fashion, some analysts say that song helped turn Christmas Eve into the most important date of the year.

The calendar may also have fed the fad. Last year, Dec. 24 fell on a Sunday, the only day when almost every Japanese worker is off. This year, the 24th is a national holiday because of the birthday of the newly enthroned emperor, Akihito.

For whatever reason, the new view of eebu has increased the Japanese people's belief that they are unique. "In the whole world," said the lead-in to a TV talk show last week, "only Japan has turned the day before Christmas into a day for sex."

Special correspondent Shigehiko Togo contributed to this report.