Tanabata (δΈε€), sometimes called the Star Festival or Festival of the Weaver, is a Japanese holiday/festival that celebrates the reuniting of the dieties Orihime and Hikoboshi.
Originally it was observed on the seventh day of the seventh lunisolar month (which could land on a variety of dates in July and August), but many celebrate it instead on July 7th of the Gregorian calendar.
Generally it is celebrated by writing one's wishes on a small colored-paper tag called a tanzaku, and hanging it on a bamboo branch. The Pokemon Jirachi is heavily associated with Tanabata.