A Japanese festival celebrated on July 7 that "celebrates the meeting of the deities Orihime and Hikoboshi (represented by the stars Vega and Altair respectively)". One observed custom during the festival is to write wishes on small slips of paper called tanzaku and hang them on bamboos. On Danbooru, the tanabata tag refers to this custom.
Tanabata originates from the Qixi festival, written the same way. When Qixi made its way to Japan, it quickly got localized and was fused with several other local traditions, notably another folklore about the Tanabata-tsume (棚機津女), hence the reading "Tanabata", and the practice of hanging tanzuku on bamboos.
Therefore, although written the same way, the terms should not be used interchangeably, Qixi applies strictly to the traditional Chinese version and Tanabata strictly the traditional Japanese version.
See also
Fukinagashi - a traditional decorative ball with streamers hung during Tanabata.
