SEASONAL means dependent on/related to a season. Happening, needed, normal for a particular time of the year. This could be a season of the year, or seasonal employment, a sports season (football, basketball, etc.), or seasonal affective disorder. The list goes on.
SEASONABLE means usual for or appropriate to a particular season of the year. Think suitable. Normal to a certain time of year. The Free Dictionary says that it’s often used as a legal term, and is usually used in connection with the performance of contractual obligations that must be completed “seasonably.” The facts and circumstances of each case define a reasonable period of time.