Look, (puts on Gieco hat), its not hard to know when to plant. Snow melts. Ground gets warm enough to dig in with bare hands. Wild plants start growing all by themselves. Even a Cave Man could do it.
The very earth under your feet tells you when its time to sow.
Nobody needs an observatory.
To the contrary. For the better part of known, written history, mathematicians and astronomers fought for building better almanacs specifically to cater for the needs of farmers. Those where extremely important researches, funded by kings and worth a lot of gold for whomever came with an edge in predicting the solar cycle exact duration. The ancient chinese emperors were responsible for deciding when to plow the earth, for instance. The power of egyptians pharaohs was tied to the prediction of the flooding of the Nile. This is DOCUMENTED history. Kepler stumbled upon his famous orbital laws almost by accident, because he was building an almanac for farmers and seafarers. My grandfather bought yearly, between the 50s and 80s, a printed almanac with dates to sow various plants calculated for the coming year. Everybody in the countryside would do the same. For the longest of times, it was literally a matter of life or death.