Tanabata is a festival of the stars, really. It celebrates the meeting of the gods Orihime and Hikoboshi, represented by the stars Vega and Altair, as for most of the year they are separated from each other by the wide river of the Milky Way, but on one night of the year they are allowed to meet, as they cross the river to be by each other's side once more.

It's quite a cute story, I think.