People of the monolithic age knew exactly how long a solar year was, because they built devices to measure such things. Stonehenge being an obvious example of something that measures the equinox. Indeed I'd suggest study of the celestial workings is the first real scholarly pursuit of mankind. Celestial navigation is highly dependent upon knowing this sort of thing as exactly as possible. In a world where compasses don't work it would have remained so for even longer. The moon is even easier to track and observe because it is so large, and because of the relatively shorter times. All you have to do to know when the new month is is go at side and have a look-see at the moon and viola. In other words this type of celestial knowledge would be among the first discoveries of mankind on any planet with astral bodies to observe.

About the week, I wasn't suggesting 7 days was too short, but rather 6 days or less. And it is not an empirical observation just an opinion. The reason being that to actually get stuff done based on 6 day weeks, especially if one day is devoted to religious observance (quite common in ancient times). At weeks of even less days the problem merely becomes that much greater.