I'm with @pseag, cold temperatures are a problem for piglets. We don't keep our sows in gestation crates, locked up tight in a shed with the piglets. It would be easier to keep the piglets warm if we did raise them that way, but it seems cruel to me. As such, we time it for spring, usually around March or April.
We aim for early spring too. We do keep our piglets with their moms but it's still better for them if it is not winter. We do tend to offset the births though, so they are delivering about a month apart.
We always shoot for late spring here. We don't usually have too many babies born each year though. Spring is our busy time in the fields too. I agree though that you don't want to have piglets in the winter months.
This website has the best calendar for breeding swine that I've found. I use it all the time to get the timing right. I usually wait to breed them until the pasture has filled out and there's plenty of forage so I don't have to spend a ton on feed.