Picture Post: Japan in bloom

Six days early, and accompanied by the sound of clinking glasses and sighs of cautious relief from meteorologists, Japan's annual cherry blossom season has arrived. The petals were once a wartime fascist emblem symbolising the transience of the soul and the closeness of death. But today, the blossoming of the sakura – as the Japanese call these ornamental trees – is one of the more life-affirming and beloved of this nation's annual rituals.

The pink flowers herald the start of spring and a new cycle of growth. For many, it is an opportunity to ponder the age-old bonds between humanity and nature, the fragility of existence and nearness of death. Primarily, though, it's a chance to party in celebration of the end of winter.

Millions of Japanese over the weekend will spread blue sheets under sakura trees, pull out beer, sake and food, and toast the spring under showers of petals. Sakura parties are one of the highlights of the Japanese calendar, and a rare break from some of the longest working hours in the world – so getting it right is taken very seriously indeed.

Because the blossoming will last only for about a week, predicting the exact day of its arrival is a science. Get it wrong and thousands of office parties and family outings go awry, weeks of planning are wrecked and millions of workers return to work disappointed on Monday morning. Shares have been known to rise and fall on the basis of a good sakura forecast.

For half a century, the Meteorological Agency has plotted the sakura front across Japan. Last year it got it disastrously wrong, forecasting the arrival 15 days early. This year the agency's supercomputer has competition from more old-fashioned methods: thousands of volunteers have snapped pictures of the sakura buds opening across the country and emailed them to a rival survey. By next week, most of the blossoms will have gone and life will be back to normal.