Summer is considered to be a fairly "hot" time of year, when people feel more perky than at other times of the year. It is possible that the rise in vitamin D in the summer, more time in social settings, and more skin in the summer months together generate more sexual interest.
But if we look at hormonal patterns, fall can actually be the time of year when sexual desire gets a boost.
Like most mammals, humans experience a kind of "biological rhythm" when it comes to fertility and other physical processes. Our hormones fluctuate with these rhythms, including hormones like testosterone, which, among other things, play a big role in sex drive.
In a study of testosterone in men and women, the researchers found that testosterone levels both fell in the summer and increased in the fall. A second study in men only found a significant seasonal peak in testosterone levels in October and November. Additional research suggests that it is not uncommon, especially in men, for testosterone to drop even in the colder winter months.
When you put these studies together, it's possible that fall is an especially tough time of year when it comes to sex drive. Another study found that men actually find women's bodies more attractive in the colder months, as women's bodies are less exposed during this time of year compared to summer time.
If you or a partner has seasonal affective disorder (SAD), the fall and winter months may not bring much sexual desire. Depression is known to lower sex drive, with decreased libido in colder months being a possible indicator of SAD.