As in other cultures, in Japan, too, seven is a lucky figure, so 7/7 makes Tanabata doubly auspicious. Tanabata aptly combines a romantic Chinese folk legend with the native Japanese custom of exorcising contamination.
Two stars, the Weaver Star (Vega in the constellation Lyra) and the Cowherd Star (Altair in the constellation Aquila), who are lovers, can only meet once a year on the seventh night of the seventh lunar month. The Vega and Altair stars indeed stand wide apart in the clear summer sky with the whole Milky Way (in Japan seen as a river, the River of Heaven) between them.
Vega was imagined to be a girl working at a loom (and therefore associated with traditional skills for women as weaving and sewing) and the constellation Aquila in which Altair stands was seen as a boy leading two cows. Because their love led them to neglect their respective duties (the lone cows wandering forlorn over the Celestial Pasture), by order of the August Gods, the pair was only allowed to meet on seventh day of the seventh month, when magpies would form a bridge with their bodies and wings they could cross.
But only on clear days: in case of rain, the River of Heaven would rise and make passage impossible so that the lovers would have to wait another whole year...
"But (as Lafcadio Hearn remarks in his version of the legend) their love remains immortally young and eternally patient, and they are happy in their hope of being able to meet on the next seventh night of the next seventh month."
In Japan the Weaver Star was fused with the legend of the celestial weaving maiden Tanabatatsume, whose task was to make garments for the Heavenly Deities, working beside the Celestial River.
Her name became the Japanese name of the festival, which was already observed at the Heian court.
The cowherd became a deity, who after visiting the weaving maiden, would the next morning take all evil contamination with him. Therefore he also purifies the world below.
So we have a Chinese legend overlaid with a Japanese one, and a lucky date overlaid with a purification festival.
To further complicate matters, Tanabata also became associated with some of the practices involved in welcoming and seeing off the spirits of the ancestors, since it fell close to the time of the Buddhist Bon Festival.
But there is nothing complicated about today's Tanabata. Since the Edo period, this important festival is just a children's pastime, although a fun one.
A week or so before the festival, bamboo branches decorated with long narrow strips of colored paper and other small ornaments are displayed in front of homes, schools, Shinto shrines, and (increasingly) in shops.
The paper strips (tanzaku) are inscribed with wishes, often of a romantic nature. I once attended a wedding party around this time, where all guests wrote a wish for the newly wedded pair on such a strip and attach it to the bamboo twigs.
The cities of Sendai (on August 7) and Hiratsuka (July 7) are especially known for their elaborate Tanabata festivals.