Tanabata (七夕) is the Star Festival held in Japan every year around July 7th (7/7, which is why kanji used are 7七 and night夕). During this time, stalks of bamboo are erected in public places and visitors write wishes on colorful strips of paper (tanzaku 短冊) which are hung on the tree. It makes a beautiful spectacle that is like a summertime Christmas tree.
Also, as this is a nighttime event tou will see many lights and illuminations, especially around shrines and landmarks which makes tanabata an especially romantic and photogenic event.
In Kyoto there is secondary Tanabata that is held in mid-August. The main attraction in this Kyo-no-Tanabata are the artistic light displays along the canal in front of Nijo Castle (Nijojo Mae station on the subway).
The exhibits are free and it takes about 30 mintutes to walk through the length of the exhibits. Some nights during the event there will be performers among the light displays (traditional shamisen players, food vendors, traditional handicrafts vendors, artists, tea ceremonies…). The display is active only during a relatively small window of time (sunset is after 7pm and the display shuts down at 9pm).