Assumption Day on the 15th August is the greatest religious summer feast in Greece. It is celebrated in all villages of Santorini and it is something that you will not miss! If you prefer a chapel, Panagia Kalou in Pori is the ideal one. It was built around 1750 by the villagers as an offering to Virgin Mary for saving their animals and crops from toxic volcanic fumes.
The greatest celebration of all though will take place at Panagia Episkopi church, which is the greatest church of the island dedicated to the most important religious feast of the Orthodox summer. It is located in Mesa Gonia and you can easily find the way by following directions from Fira to Kamari. The Church by itself is really worth visiting. Built by Emperor Alexios A Komnenos in the 11th century, it hosts the great icon of Sweet-kissing Virgin Mary (Panagia Glykofilousa), kept in a special glass case under controlled conditions. The view from the church yard is spectacular.
Tip : Traditionally on the eve, beans and fava are cooked in big caldrons!