After The Lord Buddha enlighten on the full moon day of the sixth lunar month (Visakha), he spent seven weeks near by the Bodhi Tree. Afterward, he decided to teach the Dhamma to the others, even though there were difficulties for ordinary people to understand. His former teachers were dead, the next group of people he thought of were five ascetics called "Pancavaggi", group of five, who, at that time, lived in a forest named "Isippatanamarukatayawan", the deer park near Baranasi.
On the day before the full moon day of the eighth lunar month, Asalaha, there he was. The next day, the full moon day, The Lord Buddha gave them his FIRST SERMON called "Dhamma-cakkappavattana Sutta" or the Discourse on Setting the Wheel of Dhamma in Motion. Not long after that, the head of the Five Ascetics, named "Kondanna" got "The Eye of Dhamma" and asked for ordination from The Lord Buddha. He became the FIRST MONK in Buddhism. The full moon day of the eighth lunar month became the day that honored the Sangha, or the community of the monks.
credit : http://www.dhammathai.org/e/da/asarahabucha_eng.php
Location : Ayutthaya Historical Park, Thailand