One of the six days that made up a week for the Elves, Isilya came third in sequence between Anarya and Aldúya. It was dedicated to the Moon, whose name in Quenya was Isil. There was also a Sindarin version of the same day-name, which was Orithil. In Middle-earth, the Elves' 'Moon-day' became translated as Monendei, which ultimately evolved into 'Monday'.