At the right time, opportunely, as in "The two young men desired to get back again in good season" (Charles Dickens, Martin Chuzzlewit, 1844).
Available and ready for eating, or other use; also, legal for hunting or fishing. For example, Strawberries are now in season, or Let me know when trout are in season and I'll go fishing with you. Both usages date from the 1300s, as does the antonym out of season, used for "inopportunely," "unavailable," and also for "not in fashion." For example, Sorry, oysters are out of season this month, or This style used to be very popular, but it's been out of season for several years.