Attention to the four seasons is very deeply ingrained in Japanese culture. This sense for the seasons, called kisetsukan, is particularly evident when discussing the different foods that each season brings. Shun (旬) represents the joy of eating each ingredient at its very peak, and it is an essential notion in sushi. One cannot truly understand and enjoy sushi without understanding the concept of shun.
Let’s look at kohada (gizzard shad) as an example. Kohada is what is called a "shusse-uo" (promoting fish) - it is called differently depending on its stage of development. When very young and only about 2 inches long it is called shinko, and its “shun” period is during the spring or early summer months. As the fish grows to 4 inches and reaches adulthood it is called kohada, with its peak seasonality in late autumn or early winter.
It is important to note that one type of fish is not "better" than the other, and therefore they should not be compared to each other. They are not interchangeable; each has its own season, flavors, and merits. Each ingredient's unique characteristics are what make it desirable.
The knowledge that each ingredient peaks at a very specific time each year allows the sushi enthusiast to look forward to each season and the unique flavors that it brings.