Guru Purnima is the full Moon day of the month of Ashadha, kept to honour teachers of every kind: spiritual guides, the teachers of school and music, and the family members who taught us. It is also called Vyasa Purnima, after the sage who is said to have arranged the Vedas. Students visit or call their teachers, offer flowers or a small gift, and many fast or meditate.
The date is the civil day on which Ashadha Purnima is in force at sunrise; if it is in force at the sunrise of two days, the first is taken. Because Purnima often runs from one afternoon to the next morning, the two days can both partly qualify, which is why a rule is needed. The table gives the times the Purnima tithi begins and ends in each city, with a note when they fall on the previous or next day.
Why do cities differ? The full Moon is one instant worldwide. A city in Australia sees it many hours before a city in North America, so the tithi begins and ends on a different civil date and at a different clock time. In some years that is enough to change the day a festival is listed.
In practice many people also set aside time for study or for a teacher they have not thanked in years. A calm note: the day asks nothing more than thanks. A call, a message or a few quiet minutes with a teacher’s book is a complete observance. If your family or your own teacher’s community keeps a different day, follow them.