'Lord Krishna' reincarnation of lord Vishnu. According to mythological stories and narrations. Lord Krishna was born for a reason. The reason was to kill "Kans". Yes, Kans was the tyrant ruler of the Vrishni kingdom, a place in Mathura, the brother of Devaki 'mother of lord Krishna'. Lord Vishnu reincarnated himself as Devaki and Vasudevas son during Janmastrami. That's the reason Janmastami is celebrated with enthusiasm and joy in whole of India.
Let me tell you the story behind lord Krishnas birth. Lets start from how the first time lord Krishnas birth reason was told to Kans. The day of Devaki and Vasudevas marriage, Kans out of the love he had for his sister decided to be the bride and groom's charioteer for the day and drove the chariot bearing Devaki and Vasudeva out of the wedding hall, while a voice from the heavens boomed! telling Kans that Devaki's eighth child would be the reason for his death. Listening to this Kans got furious, and wanted to kill Devaki then. That's when Vasudeva stopped him, pleading for his wife's life promised Kans that every time Devaki would give birth to a child it would be handed over to him. Kans agreed to this and put Devaki and Vasudeva to house arrest.From then onwards every time Devaki gave birth to a child, Kans was informed about it by the guards and immediately Kans would kill the child. Six children of Devaki suffered this death. When the 7th child was born to Vasudeva and Devaki, they decided to save him from cruel Kans and Vasudeva managed to escape the prison, guards were asleep when Vasudeva took his child to his second wife Rohini in Gokul and managed to get back in the arrest without being caught. Rohini named the child 'Belram'.
Now was the day when Devakis 8th child was born "lord Krishna". The eighth child born on the eighth night of the Shravan month, It was raining heavily and the skies thundered as if the Gods were trying to bless the new born child. Then the miracle happened, Vasudeva got freed of his chains and the prison door opened, Vasudeva found the guards fast asleep and he decided to escape with the child and leave him at his friend Nanda's place in Gokul. Vasudeva Picked the child and placed him in a basket, carrying the basket on his head he walked his way to Gokul. Gokul being on the opposite bank of the river Yamuna due to the thundering rains the river was in a state of turmoil. Vasudeva,wondered how he would reach Gokul he prayed to the lord for showing him a way to save his child and the miracle happened the river parted and made way for Vasudeva, walking through the river Yamuna he reached Gokul. When he reached Nanda's house to leave his child he realised that Nanad's wife had given birth to baby girl. When Nanada and Yashoda were asleep, Vasudeva placed his child in the cradle and took Nanda's baby girl along with him back to Mathura. The guards were still asleep, seeing this Vasudeva went back to the prison. As soon as they reached the prison the doors got locked and the baby girl started to cry, the child's cry woke the guards and Kans was informed about the eighth child's birth. Kans came to the prison to end the eighth child's life as well, Vasudeva and Devaki prayed to Kans to spare their child not to harm her, Kans with the fear of death picked the child and was about to bang her down on the floor when she flew away from him in the air and told Kans that the child whom he wanted to kill is safe in Gokul and he cannot change his destiny,with this the child disappeared.
The importance and significance of the celebration of Janmastami is what the story narrates. The verses from the Holi Bhagvat Gita (narrated by lord Vishnu) says that whenever there will be predominance of evil and decline of religion i will reincarnate to kill the evil and to save the good. This is what the story narrated, lord Vishnu reincarnated as Krishna to kill the evil and save the good. Kans was the evil in kings disguise. This is the significance of the celebration of the Janmastami festival. With the belief that good will win over evil. Lets prepare for the much awaited Janmastami.