Nag Panchami is a significant traditional festival observed in Nepal, as well as in certain regions of India and neighboring countries. It revolves around the veneration of snake gods, or Nagas, and carries profound cultural and mythological importance. As you mentioned, according to the legend, Nag Panchami is linked to the belief that Nagas could control rainfall and once withheld rain over Nepal. The king of that era, who was also a Tantric, employed his mystical powers to pacify the Nagas and bring rain to the region. This event represents a triumph over the Nagas' influence on the weather. Nag Panchami is celebrated as a festive occasion, acknowledging the importance of Nagas.