Just as a worldbuilding factoid - in many places, winter is actually one of the best times to attack. Rivers are frozen over, the terrain is sturdy and good for long marches, and there is little else to do - as in, no crops that need tending.
Summer is another good time, as the weather is less adverse, and, again, you can move your bands through ground that isn't wet and slippery. Most of the work, sowing and reaping, is done during spring and autumn respectively. If the day-job of your warrior band members is agriculture (and if we're playing feudalism, it is), those are the times when they'll least want to actually fight for you.