The 1st day of Paryushan is called Amari Pravartana (also written Amaripravartana). It marks the vow to refrain from violence and harm, beginning the period of intensified self-purification and reflection. In Śvetāmbaras, this kicks off the eight-day observance; Digambar Jains may have a slightly different structure, but Amari Pravartana is commonly accepted as the first day in many Jain traditions. You can read more here