Asadh 15 (Nepali Date) today marks the end of rice harvesting period in Nepal. Rice is the main staple food of Nepalese people in the morning and in the evening since the ancient times. So some sort of celebration is allocated especially to the farmers to mark the end of harvesting season and pray for the production of good quality of crops later. Its the main source of livelihood and income for farmers living in the remote areas. The rice is exported to urban areas of Nepal.
The tradition of National Rice Day is usually celebrated by eating curd or yoghurt with beaten rice. Other caste people like Tamang, Newari, Bhote and so on have the tradition of getting drunk with Chyang (made by fermenting rice) and Raksi (like gin). After harvesting for a day, at the end they play music and then dance and play in the mud. People will hit each other with mud. You play dirty during this occasion.
This festival also welcomes the season of monsoon. Farmers rely on this monsoon rain for irrigation purpose. This festival was put in place in 2005 with the view of expanding the rice cultivation throughout the Nepal. Until now, its not enough to feed every Nepalese and most of them are exported from India to meet the needs of Nepalese. I personally had never went to witness this festival. I watched the video on news portal and TV. They had great fun. If you think why Nepalese made this rice plantation such a big festival then there is one and only one things you need to know: about 71% people are engaged in agriculture here in Nepal, which is another third highest source of country's economy. The first comes from remittance and the second from tourism.