Population Migration / Travel
After a series of small scale migrations, the permanent population on Mars passes 1000 people. They are spread across 2 bases, Utopia and Phoenix, and are just the seed for what is to come.
While not an incredibly important date, this is often seen as the marker of when Earth finally had a true backup. While it would be difficult, it is likely that any event decimating Earth's population could be ridden out on Mars, keeping humanity alive. At this point, the population of Mars is actually 1072. 2 ships came down at the last window, bringing 180 people. It is spread, with 528 at Utopia, 319 at Phoenix, 89 at Olympica, 70 at Marinaris, with the remaining 66 on various long term expeditions, like the 24 going up Olympus Mons.