Nature is something that we seek to control and generally cannot. Weather is one such aspect of nature. What if you grew up in a culture that believed that weather was more than just nature interacting with itself. In the past people have believed thunder and lightning was the anger of the gods. This is a similar type of belief.
A lot of prophecies deal with the day someone is born, but what if you took something generally harder to know or control, when someone was conceived. People wanting a more powerful child might try to conceive that child on a harsh weather day. People wanting to give their child an easier time might try to avoid harsh weather, might even only try for a baby on a sunny day.
What kind of society might that bring about, one that was obsessed with the weather, trying to give their child the type of life they wanted for them before they are even born. An ambitious child have to work that much harder because they were conceived on a sunny day, but once they gained power, it might be easier to keep. A lazy child might have to fight to have a day off and to just relax. What happens when what the child wants doesn’t align with what their parents want in a society that believes this? And is it all real? Or is it all in their minds?