There are many determinants off demand for a product. Clearly the price of the product influences the demand, but many other factors may be influential in determining the demand. The seasons are one such determinant.
For example or demand for heavy and warm clothing is greater in the winter than in the summer. Likewise, there is a greater demand for cold drinks and ice cream in the summer as compared to the winter.
There are many examples that we may think of that will suggest that the seasons are a determinant of our overall pattern of demand.