The History of Losar
Significant incident that leads to the start of Tibetan Losar happened during the 9th Dynasty under the rule of King Drigum Tsenpo. During that period, Tibetans were not Buddhists. They were mainly the followers of Bon, a religion that was quite similar to Shamanism. They lived in nature, and they revered nature, sky, river, lake as God. It was believed that up to the 9th Dynasty, all Tibetan kings were sent by the Gods. They came to the earth using a thin string made of light. In Tibetan, this string is called དམུ་ཐག (pron: mu-thag), which means divine string. The kings came down to rule the Tibet during the day, and during the night, the light string would come down again allowing the kings to go back to the heaven. At the time of their deaths, the kings used the string to go back to heaven, and the string was pulled back. The king never came down anymore. That would be the end of the king's reign.
However, trouble came during the 9th Dynasty. Drigum Tsenpo was a little bit naughty. He challenged his own ministers and got into arguments with them. The angry king wanted to fight the ministers by sword. The ministers tried to persuaded Drigum Tsenpo not to fight because he was a God while they were only ordinary beings. However, the king insisted to fight, and so they conducted fights with sword. During the fight, the string was accidentally cut off by the sword, and never came back anymore. Sadly, during the fight, the king was also accidentally killed by the sword. Hence, from the 9th Dynasty onwards, Tibetan kings were mortals. When they passed away, their bodies were left and tombs were erected in memory of them. The first to the eighth kings did not have tombs.
Despite suffering the unfortunate death, Drigum Tsenpo came with a legacy. During his time, he always had an apricot tree nearby wherever he stayed. The tree grew flowers that bloomed during spring. It was usually wonderful time for people. The king then declared that whenever the flower bloomed, they were going to celebrate the time as a new spring. They did not call it Losar at that time, but chidsar (དཔྱིད་གསར་), that literally means new spring. During the time, animals were sacrificed to the God as part of the celebration. People gathered to dance and eat. This is how new year celebration started.
When Tibetan astrology was discovered, people matched the celebration day with the beginning of the year since the dates coincided, and since then chidsar was called losar (ལོ་གསར་), that literally means new year. From the 7th century onwards, Buddhism was prevalent in Tibet. Many people converted to Buddhism. Consequenty, they did not sacrifice animals anymore, performed more prayers and offerings, and celebrated thanksgiving. Changes were made to suit the Buddhist culture, e.g. prayer flags were erected.