It simply depends on what you sell. You have to learn the trends for each product industry and how they operate world wide. There is a science to it, and you have to study the industry you are going to specialize in. There is no one easy quick fix answer to your question because products come and go over time, some short, and some long.
We sell a lot of books year round to public, school, and university libraries all year long. Text books are always heavy at semester start times. Specialty Trade instructional manuals are good all year, where novels aren’t worth anything because they are printed by the 100,000’s.