By the 1970s amateurs had noticed that all 110 Messier objects could be observed at low northern latitudes over the course of a night in mid-to-late March.
You’ve probably heard of Charles Messier’s catalog of celestial deep-sky objects for 18th century comet hunters. When they stumbled upon an unknown faint fuzzy object, they’d consult this list to see if it was a known object. It was first published in 1774 and expanded with help from fellow observers in that century. Astronomers and amateurs in the early 20th Century rediscovered it, added a few new objects, and made corrections. There are now 110 “M” objects in the modern catalog.
Every sky chart labels the location of these objects with the letter “M” and a number. For instance, the brightest Messier is M45 — the famous naked-eye Pleiades open star cluster. A third of the way around the sky you’ll find the dimmest: M95 — a galaxy of magnitude 10 or 11 (depending on the source) in Leo.