Bananas are of course available pretty much all year round thanks to massive import and production methods.
Choosing bananas is a bit of an art, you need to know when you want to eat them because they ripen very quickly. But did you know it also depends on how you want to cook the banana?
Baking - If you want to bake bananas then you want the brownest and softest banana. As bananas age they release sugar an and aroma, which is exactly what you want in cooked bananas.
Eating Right Away - This is totally a personal preference thing, BUT, you should be aware that bananas have more flavour the browner they are. A green banana can taste almost watery.
Always store bananas at room temperature - never put them in the freezer unless you want to use them for baking. Bananas release 'esters' which trigger chemical reactions in other bananas. If you put a super-ripe banana next to a green one the esters will make it ripen much faster (particularly in an enclosed space like a bag).