Monendei

From the earliest times, the Elves had dedicated a day of the week to the Moon. They named this day Isilya in Quenya, or Orithil in Sindarin. The Númenóreans adopted the Elves' system (with some adjustments) to their own use, and ultimately transmitted these to Middle-earth. Monendei ('Moon-day') is an archaic form in the Common Tongue of this name, and by the time of the War of the Ring, this would be worn down to the familiar 'Monday'.