The seasonality of an ingredient refers to the period in which the ingredient is available or is at its best. Some ingredients, such as asparagus, are only available several months of the year whereas others, such as cabbages or carrots are available fresh all year long.
These days, there are many ways to get ingredients out of season. Some ingredients, like apples, will simply keep for a very long time, without much extra effort. Others, such as peas, can be frozen directly after harvest to make them available all year round. Other options include growing vegetables and fruit in greenhouses to simulate seasonal conditions or flying produce in from parts of the world where the seasons are reversed (ie. the season for asparagus on the southern hemisphere is six months later than on the northern hemisphere).
The following tables show the seasonality per ingredient for each month.
As the seasonality of ingredients depends greatly on the climate in which they're produced, these tables are intended to be used as a loose guideline. Depending on the location, the season may be longer or shorter.
When buying ingredients, check the country of origin if possible. Locally produced vegetables that are far out of season are likely to have been grown in greenhouses.
Turkeys are often eaten for thanksgiving in the fall just after fattening up in the summer. Pigs and other animals are also often slaughtered in the fall after freshly fattened up. Ham is traditionally paired up with the autumn apples. Salmon are popular according to their spawning cycles. In Portugal sardines are a famous festivity as Saint Anthony's day in the summer days of June.
Other examples include certain cheeses and alcoholic beverages. Feta cheese, for example, is aged for at least 3 months at certain conditions. Winter lagers also have to be fermented at lower temperatures for several weeks.