This information is very approximate and can change from year to year a bit. There's another issue, you could get new crop Costa Rica, for example, in December or January, but would you want to? Probably not!
Those shipments are lower grown coffees, lower altitude farms where coffee cherry ripens sooner. These are not going to have the cup quality of higher grown coffees, and will lack bean density and brightness. And sometimes coffee is "rushed" through production for market reasons, ruining any cup quality it has. Coffee needs careful, timely processing, and it needs to "rest" in it's outer parchment layer for 30-60 days at least after drying.
Even at the good farms, there are still early, early pickings of ripe cherry that are not usually the best coffee the farm will produce. For that you need to receive a lot of "heart of the crop" coffee, the mid-crop pickings when the cherries have uniformly ripened. The best lots of a farm, their "reserve" lots come from these pickings.
In our Costa Rica example, those would arrive in the U.S. starting late March through June, in most seasons. There are always exceptions, and there can always be a great coffee the arrives out of sync with the crop cycle. Sometimes the first Harar arrivals are best, oftentimes not. Sometimes the last Harar arrivals are best. Cupping is the way we find out.