七夕 Evening of the Seventh
Tanabata is a yearly Japanese holiday that takes place on July 7th. "It celebrates the meeting of the deities Orihime and Hikoboshi (represented by the stars Vega and Altair respectively)."  The actual date of Tanabata may vary from region to region.
Part of the celebration involves writing wishes on slips of paper and hanging them on a tree.
The holiday is based on the Chinese myth of the weaver and the ox herder in heaven being separated except for on Tanabata which is the one day of the year when magpies form a bridge for them to meet.
While the myth is based on Chinese constellations, the main stars involved are the famous Summer Triangle which are Altair, Deneb, and Vega.
Vega is known as Orihime 織姫 or the weaving girl in the myth.
Altair is Hikoboshi 彦星 or the cowherd star who is separated from the weaving girl. (In Arabic, Altair means Eagle. The protagonist of the first Assassin's Creed game is named for this reason.)
Mentioned in shows
Densha Otoko famously begins its TV series on Tanabata. They tell a parody of the myth that Tanabata is based on. Then the scene fades to Densha and Hermes against a starry sky, foreshadowing their future romance and the gulf between them.
Den-O's second rider Kamen Rider Zeronos is completely themed after this holiday/constellation. His base form is called Altair Form. Both this form and ZeroLiner are based on cattle. His partner Imagin is named Deneb (after the star) and is themed after a magpie and a warrior. When they combine, the form is called Vega form. In drill mode, ZeroLiner's drill looks like a weaving spool.
The Chinese version of the myth is mentioned in the movie Gold Mountain.