How the date is fixed
Guru Purnima falls on the full-moon day (Purnima) of the lunar month Ashadha. The date is the day on which that tithi is running at sunrise (the usual rule, called udaya tithi).
About
Guru Purnima is the full moon of Ashadha, in June or July, the first full moon of the monsoon. It is also called Vyasa Purnima after the compiler of the Vedas, and Buddhists mark it as the day of the Buddha's first sermon at Sarnath. The tithi at sunrise fixes the day. Ascetic orders begin their four-month chaturmasa retreat from this full moon, which is why the day is associated with the teacher–student relationship.
How it is observed
Students and disciples offer respect to their teachers, and ashrams hold Vyasa puja and readings.