Krishna's last day was during the Yadavsthali war. After the Mahabharata, most of the powerful families were destroyed, and only a few remained, with the most powerful being the Yaduvansh and Pandav vansh.
Because of the friendship between the Yadavs and Pandavas, and thanks to Shri Krishna, the Yadavs started to believe they were the strongest group. They began to act spoiled and careless. During this time, Krishna knew that his clan was going in the wrong direction, but he couldn't stop it.
After the Yadavsthali war, all the male Yadavs died. Even Balram, Krishna's beloved brother, left for the cosmos. Krishna witnessed the death of his loved ones and relatives. Although he was a god with a strong mind, seeing all this made him a bit sad. He felt that his work on Earth was finished, and it was time for him to return to the cosmos.
