Mahavir jayanti is celebrated to commemorate the birth of lord mahavir, who was the last tirthankara, also known as a spiritual teacher of jainism. He was born in 599 BCE, in kundagrama also known as bihar today. He gave up his royal life at the age of 30, to follow the past of spirituality and become a monk. He teached the core principles of jainism, which includes ahimsa [ non violence], satya [ truth] asteya [ non stealing ], brahmacharya [ chastity ] and aparigraha which notes non possessiveness. Mahavir jayanti is celebrated through status, mahavira also called as the yath yatras. The people visit temples, do prayers and some also fast. 

He preached ahimsa, but not only that, but he also lived it fully. He believed that any kind of harm to animals, plants, humans, or living beings is a sin.