The name used in the Shire at the end of the Third Age for the sixth of the seven days of the week, following Hevensday and preceding Highday. The name had evolved from an earlier form, Meresdei, which meant 'Day of the Sea'.
That the water-shy Hobbits should have a day dedicated to the sea is a pecularity explained by the history of the word. The original Elvish week only had six days, but in Númenor, a seventh was added. As a great nation of mariners and explorers, it was natural for the Númenóreans to make this a 'Sea-day', which they called Eärenya in Quenya. In Middle-earth, the tradition of a seven-day week continued, but in the Shire the name was translated into the Common Tongue, and so the Shire-hobbits acquired their own 'Day of the Sea'.