Monday follows Sunday because the planetary hour system that generates the week always advances the first hour's ruler three steps along the Chaldean order, the sequence of the seven classical planets arranged from slowest to fastest as seen from earth: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Three steps on from the Sun lands on the Moon, so the Moon rules the first hour of the day after the Sun's day, and the day takes its name accordingly. Like Sunday, Monday kept its Latin planetary name directly into English, since mona, the Germanic word for the Moon, needed no substitution the way the days named for Mars, Mercury, Jupiter and Venus eventually received.
The Moon's Latin name, Luna, and its Germanic equivalent both describe the same visible body, which is why dies Lunae, lundi, lunedi and Monday all preserve a recognisably lunar name across languages descended from very different branches of Indo European, unlike the days whose Latin planet was replaced by an unrelated Germanic god's name.
Sources Norse Mythology: A Guide to the Gods, Heroes, Rituals, and BeliefsJohn Lindow with Deutsche MythologieJacob Grimm