Tanabata is an event that originated from the legend of two stars called Ori-hime and Hiko-boshi. It is held on July 7th every year. Tanabaka is also known as the Star Festival in English. Orihime represents Vega, Hikoboshi represents Altair.

The Story Outline
Orihime and Hikoboshi fell in love and were so crazy about each other that they could not focus on their work. The heavenly gods got angry and separated Orihime and Hikoboshi onto the different sides of the Milk Way, or Ama-no-gawa in apanese.

However, the heavenly gods felt sorry for them and allowed them to cross the Ama-no-gawa to meet each other once a year on July 7th.

On this day, many people pray for good weather on July 7th, because if it rains, Orihime and Hikoboshi will not be able to meet up due to the rising water levels of Ama-no-gawa.
It is said that Tanabata is a special day for lovers because it is the day where lovers reunite.

【Reference】https://eikaiwa.dmm.com/blog/40355/