Kevalgyan is the state of mind of a person in meditation, when he is believed to have achieved the supreme knowledge. The spiritual aspect of all the religions believes that all the humans have the tendency to achieve this supreme knowledge, which lies inherent in our minds.
According to Jainism, a person who has achieved this phase is known as a Kevalin. The one who has become a Kevalin is also known as having attained the omniscience - a man having the real knowledge of all that exists, wherein others are believed to be having just the partial knowledge.
Every year, on the tenth day of the Shukla Paksh of the Vaishakh month this day, when Lord Mahavira attained the Kevalgyan, is celebrated. He had become the omniscient, the one who knows everything, on this day. This year, the day falls on the 25th of April, 2018.
There are two sects in Jainism. One is the Shwetambars and the other Digambars. However, both the sects differ in their views about some of the characteristics of a Kevalin. While the Digambars believe that a Kevalin, i.e., the one who has attained this level of knowledge, does not feel the need for eating or drinking, he experiences neither hunger nor thirst. On the other hand, the Shwetambars believe that they do feel them.
However, they agree on the fact that the last person to become a a Kevalin was one of the disciples of Lord Mahavira.
Jain texts speak about the various stages that a person in his lifetime may go through. He starts from that of an unaware person and may attain the last stage, which is that of a Kevalin. The first is believed to be the worst stage known as Mithya Drishti, which is the stage of the wrong doer. Then, slowly he moves towards the second stage, believed to be a little less worse. This is the Sasvadana-Samyadrasti, the stage when the person has at least some taste of right belief. Then, follow the Misradristi and Avirata Samyagdristi.
While the common man may reach the fourth stage, crossing it and going beyond is what only the one who has taken strict vows the Jain ascetic life can do. These vows are known as the Mahavratas.
Jain Texts - Uttar Purana and Harivams Purana mention about Lord Mahavira attaining the supreme knowledge. Lord Mahavira had crossed the twelve stages of life and had attained this supreme stage after a lot of difficult penance.
Lord Mahavira once reached the village named Jarmbhika and sat in a park, behind the river Rijubaluka, sitting in a squatting position. It had been twelve years, five months and fifteen days, and then came the day when he attained the Kevalgyan, the knowledge of all. He became a Jina.
He practised deep vows, sacrifices, and meditation for about twelve-and-a-half years. He stayed in jungles and unknown places, secluded himself from the society and faced various austerities as well.
Starting from fasting for two days initially, he increased the number of fasts to those lasting for six months.
It was the tenth day of the Shukla Paksh in the Vaishakh month, that he attained this stage. Since that day, every year this day is celebrated as the day of Lord Mahavira's Kevalgyan. Lord Mahavira's first sermon after this has been recorded in the book named Agamass.
Again, there is a difference in opinion of both the sects about Lord Mahavira's journey after this stage. While the Shwetambars believe that he went about travelling and educating the disciples and followers for the next thirty years, after having become a Kevalin, the Digambars on the other hand believe that he just sat in his Samavasaran, and gave sermons to his followers after this phase of life.
Lord Mahavira has been described as the one who can see all in the Acharanga Sutra. Another text - Sutra Kritanga - talks about the other qualities of Lord Mahavira as well.