What does the tanabata festival celebrate?

Tanabata, the Star Festival, is an event based on a legend in which two lovers, Kengyu (the star Altair) and Shokujo (Vega), who were separated by the Milky way, meet just once a year on the night of July 7.
Originally from China and modified into its Japanese form, the event spread from the imperial palace to ordinary people.
Bamboo branches are set up in the garden and decorated with strips of paper on which people write their wishes.