Traditionally this day is reserved for Guru Puja or for giving thanks to the Guru in your life. On this day disciples offer Puja or pay respect to their Gurus. Guru refers to spiritual guide who enlighten disciples by his knowledge and teachings.
Guru Purnima is also known as Vyasa Purnima and this day is commemorated as birth anniversary of Veda Vyasa. Veda Vyasa was the author as well as a character in the Hindu epic Mahabharata. Vyasa is said to be the first Guru in the long lineage that brought us B.K.S. Iyengar.
Below is a link to Guruji's talk on the Yogic Mind from Guru Poornima in 2009