For me that decision would depend on whether I had any indoor barn/shed facility I could use when the weather gets really bad in the winter. If you are entirely outdoors, as we are, and on heavy clay soil, several days of torrential rain can create havoc with your ground if pigs are constantly trampling through it, creating a glutinous slurry that remains months after the rain has stopped. If you are able to get them inside during the worst rain downpours, the ground gets much less mashed up and can dry out more quickly. Cold temperatures don't seem to be an issue with pigs, they cope perfectly well with that. Other reasons I prefer spring/summer pigs are - the grass lasts longer which they nibble at constantly, better digging/rooting conditions, sunbathing, you can walk through the plot and get to the ark without sinking/getting completely stuck in mud, you can choose where to have a wallow instead of the whole plot being one, and generally, you get to see more of the pigs as it's nicer for humans being outside in summer.