In Jain tradition, “Atham / Attham” commonly means a fast of 3 continuous days. The name is used in practice to refer to three consecutive upvas, with parna on the 4th day. So although the word may sound like it should mean “eight,” in Jain usage it is traditionally understood as a 3-day tap. ( jainknowledge.com)

If you want, I can also explain the difference between Attham, Chhath, and Ayambil in Jain fasting.