Traditionally, oysters are eaten mainly in autumn, winter and early spring. It's the real oyster season. In summer, the sea becomes too hot and the oysters risk becoming infected with poisonous algae.
At the same time, summer is the period where oysters spawn. And the spawning means that the oysters turn white and milky. If the oyster is milky, its metallic and fresh taste disappears and it gets a less attractive taste instead.
The rule of thumb - if you are in the northern hemisphere - is that you only eat oysters during the months that include an 'r'. So not in May, June, July and August, but preferably in January, February, March, April ... September, October, November and December.
The rule does not apply if you for example are in Australia – here May, June, July and August are the cold months where the oysters are at their best.
The tendency, however, is that you can increasingly also eat oysters in the summer. Many oyster producers have their oysters going in basins that ensure they are cleaned and detoxified, even if they should have been in contact with toxic algae.
At the same time, oysters are increasingly being produced which are not fertile and therefore do not lay eggs. The goal of creating non-fertile oysters is precisely to avoid the milky character of the oyster.
Through this, the oyster producers try to turn oysters into a dining which can be eaten both in months with and without 'r'; all year round. Read about why Alabama oysters from US also can be eaten during the summer.
However, oysters tend to taste differently in different parts of the year. And the best part of the season with regards to taste is from November to January. That's the period where the oysters are fattest and most stuffed. They lose weight during the winter and lives of their reserves, so in early spring they become thinner and leaner.