Festival of cows- Gai Jatra today

Gai jatra is the festival of cows (gai means cow and jatra means festival in Nepali) whereas in Newari, it is called as Saparu (सापारु). It is celebrated next day after the Janai Purnima.
During the festival, cows are marched in the streets and generally celebrated in the Nepalese month of Bhadra (August-September). Peoples also distributes food to others. The festival of cows is one of the most popular festivals of Nepal.
According to the traditions since times immemorial, every family who has lost any relative during the past year must participate in a procession through the streets of Kathmandu leading a cow. If a cow is unavailable then a young boy dressed as a cow is considered a fair substitute. In Hinduism, a cow is regarded as the most venerated among all the domestic animals. It is believed that the cow, revered as a holy animal by Hindus, will help the deceased relative’s journey to heaven.
When and why this was started?
Back to history, when the King Pratap Malla lost his dear son, his wife the queen remained in grief. So the King tried various effort but wouldn’t make his beloved queen smile. So he announced to his public that those who have lost someone dear make a procession with some funny attire and activities so that they can forget their grief. Then queen was shocked to see a huge group of people and realized that she was not only one to lose the dear ones. Everyone has lost someone in their family and life. So to share grief of losing someone and to forget about the lost and to pray for the demised person, Gaijatra is celebrated till now.
Those people who have lost someone in their family and life are supposed to participate in Gai jatra but some participate just for fun too.
Similarly, in Newar culture, there is also tradition to give or share small green beans soaked in water for overnight and mixed with spicy ingredients, pear along with other stuffs like sweets to each other in the community.