Now, of course ...at least if you haven't already started one. Composting can occur all year long in almost any climate.
The spring may be the easiest time to compost. Rising temperatures are good for the micro-organisms in the pile and they should be thriving. The optimum temperature for breaking down organic matter is between 140 and 160 degrees F. Some compost bins are designed to allow to good air circulation, but if your pile is stagnant, be sure to turn it so that air can reach the deeper layers.
In the summer, you should have a good amount of "green" nitrogen rich grass clippings to add. Layer the grass with "brown" carbon rich matter like early crops, straw and wood chips to balance the mix and prevent matting Be sure to keep the compost pile moist as the weather warms up or the composting process will slow down.
In fall...if you start composting at this time, your bin will be a great place to put the dead leaves you rake up, rather than bagging them and sending them to a landfill. Tarp the pile if necessary to prevent the compost from becoming too soggy.
As the winter months roll in, you may notice the composing process slows down. Don't worry—as long as temperatures stay above freezing, your compost will continue to thrive. In the spring, you will be able to use the compost generated through the year to supplement your new garden plantings.