It is one of the chariot festivals of Nepal. Hindu people believe that Rato Machhendranath is a god of rain.
Both Hindu and Buddhists worship the deity in Nepal. Hindus regard him as the reincarnation of Lord Shiva. The Buddhists regard him as the Tantric reincarnation of Avalokiteshvara.
more at http://www.imnepal.com/rato-machhendranath-jatra-chariot-festival-nepal/
My video is at DLive