Days Of The Week (Roman Names Of). Sunday was called Dies Solis, or the Sun's day; Monday, Dies Lunae or the Moon's day ; Tuesday, Dies Martis, or Mar's day; Wednesday, Dies Mercurii, or Mercury's day ; Thursday, Dies Jovis, or Jupiter's day ; Friday, Dies Veneris, or Venus's day; and Saturday, Dies Saturni, or Saturn's day. The reason they were named thus, was because they considered each of these deities to preside over the day dedicated to them