Charles Messier was an 18th-century French astronomer whose primary goal was to discover new comets. In the process of his comet searches, Messier occasionally found objects that appeared comet-like in the small telescopes he used, but which were not comets. They appeared to be permanent parts of the night sky (like the stars), but he was unaware of the true nature of most of them. To avoid confusion in future comet searches, Messier compiled a catalog of these non-cometary objects.
Messier Marathon is a term describing the attempt to find as many Messier objects as possible in one night. This opportunity occurs once every year, around mid- to end-March; the best time to try is of course when the Moon is near its new phase. Best dates for MM
Constellation Names, Abbreviations and Pronunciation - Sky & Telescope