I remember "learning" about stocks in my high school econ class. After a few days of talking about them, my teacher set up a stock picking contest for the students, many of whom had gained no understanding whatsoever of the market during the tutorial. Anyway, one the biggest mistakes I remember people making - at least in my mind at the time - was thinking about what season it was and then was becoming to pick their stocks. "It's getting warm, so who makes swimsuits? was a question one student pondered).
Good investors look at trends, and I guess the whole four seasons to a year, hot-warm-cold-warm-hot-warm-cold-warm (etc) thing was the easiest trend for lazy high schoolers to digest and analyze. Unfortunately, it's tough to make money with that level of logic. Why? The markets already know that swimsuit maker is going to make more money in the Summer then the Winter, so it's already priced into the market (meaning the stock price already reflects this understanding).
So why am I bringing this up 6 years later? Well, for one, I organized a stock picking contest at my old job, and the same thing happened again. This time by college graduates with at least a basic understanding of the economy and stock market (some of them kept personal portfolios). Nonetheless, they made their decisions based on fairly basic trends, including those damn seasons.
I guess the point of this rambling post is that there are many levels of analyzing trends that effect the stock market, and there are people out there who choose some of the simplest trends to make their decisions. While I use the seasons as an example of something so obvious it's already priced into the stock price, also note that if enough people buy or sell based on such trends, the stock will move accordingly and there will be money made on the upside or downside; so even the most sophisticated investor may find instances that it makes sense to buy or sell based on seasons (though really he or she would be trading based on HUMAN trends, which are probably the most complex!!).