UNLESS IT COMES in a heavy downpour, rain does not prevent a prime minister of Japan from holding his traditional cherry blossom party. So Takeo Fukuda slogged his way dutifully through Tokyo's Shinjuku Gardens this week, waving at several hundred invited guest huddled under umbrellas and tents to escape a steady drizzle.

His grim determination to celebrate the blossoms was matched by the guests'. While the rain fell, they took group pictures under the trees and crushed each other to get at the sake, beer and sandwiches in the tents. They walked down muddy paths to examine the health of the Shinjuku gardens' 34 species of cherry trees while a military band tried to lift the dampened spirits with waltz music.

The sodden scenes was testimony by one of Japan's most enduring traditions, cherry blossom time. The histories say the Japanese have been fascinated by cherry blossoms since the 5th Century. A great web of mysticism has grown up about them and they have been a marvelous source of symbols for poets through the centuries. As Fukuda demonstrated, politicians still pay homage at this time of year.

Their special place in Japan is shown in many ways. There is a venerable literary tradition holding that a woman's beauty may be compared to a cherry blossom, but not to that of a plum blossom or any other flower.

The Japanese word Hanami literally means flower-viewing, but it is correctly used only to describe the viewing of cherry blossoms. In the old days, when Hanami was a prerogative of the aristocracy, women owned gowns to be worn only on those early April days when they strolled among the cherry trees.

The metaphysics of cherry blossoms is the theme of many plays, novels and poems. Their brief period of blooming is supposed to represent the fragile and transistory nature of life, and their sudden fall after days of glory is a symbol of death. The moral is that life should be lived brightly and with spirit while it lasts.

BUT. STRANGELY. the delicate, pale pink blossom is most often associated in Japanese literature and folklore with the samurai, the warrior of old who hacked his way through bloody battle after bloody battle and who often ended his own life in glory ritual suicide.

How this particular metaphor got unleashed is not clear. But in the popular mind, the brief, hard life of the samurai is identified with the cherry blossoms' single week in the sun. It follows, then, that at season's end the scattered petals represent the bodies of dead warriors strewn on the battlefield.

The "live life brightly before it ends" theme is celebrated by modern Japanese with family parties, strolls in the parks, and monumental rounds of sake-drinking. Last Sunday, before the rains came, more than a half-million people crammed into Ueno Park, one of Tokyo's favorite blossom viewing areas. Thousands of others celebrated with booze and picnic lunches at Yasukuni Shrine, Japan's memorial to the war dead, or strolled along a most by the Imperial Palace.

Some groups stake out picnic grounds by roping off squares. Mats and blankets are spread out for a day of drinking and singing. In the old days, roving troupes sang and danced around the picnic spreads. Today the entertainment is more likely to be American rock music on transistor radios. By nightfall the parks are full of rubbish and the air is heavy with the stench of overripe food left behind.

SUCH REVELRY has been going on for years and Tokyo takes in stride the many scenes of public tipsiness in the parks and on the streets. By the standards of American college beer blasts, they are pretty mild events, and Japanese policemen are not inclined to interfere. The tradition is too old, for one thing. The parties have occurred every spring for centuries. Except during World War II and a few years afterward when Japanese were hard-pressed to accumulate enough food and drink.

By several accounts, the parties are more frequent and lively this year than in the past, for unknown reasons. Some feel that with Japan still in a recession and unemployment becoming more serious every day everyone wants to drink a bit more to ease his frustrations. That's the somewhat whimsical theory of Yasumasa Tanaka, a professor of social psychology at Gakshuin University (whose school symbol is a cherry blossom). The campus has never witnessed as many parties as this year, he said.

Others take a less serious view of the whole cherry blossom spectacle. They thing the revelry hasn't much to do with either the Samurai of old or current economic conditions.

One Japanese reporter, an experienced participant in such boozy celebrations, observed this week that his countrymen also like to drink a lot in the fall when the leaves of trees turn red and gold. Perhaps, he said, people just want an excuse to drink sake.