Our calendars are based on the motions of the Sun and Moon. The phases of the moon provide a convenient way to keep track of time.
Phases are easily visible and distinctive.
12 lunar months is 354 days. The oldest recognizable ancient calendars are lunar calendars. A particular difficult of pure lunar calendars is that the 354 d lunar year is 11 d short of the 365 d solar year. This causes the seasons to drift among the months. The Babylonians built a complex, but very precise hybrid luni-solar calendar based on the Metonic Cycle.