When possible, I plan for foals to be born in mid-March. It's still fairly early in the season so they'll be good-sized yearlings, but by the middle of March we're usually out of the worst that Winter throws our way here in central Kentucky.
Until two years ago, I never gave too much thought to foaling complications. I'd read plenty about breached births and retained placentas and cast deliveries and parturition colic, but was fortunate to have experienced only straightforward births in the past. This afternoon during a veterinary appointment I discussed some foaling issues with my vet, and we reminisced about a "rejected" foal that we dealt with in 2007. (I blogged about it at the time: Making a New Mom for a Rejected Filly.) Ever since then, I've paid special attention to the potential for problems during foaling because I want to be prepared regardless of the situation.
An article from The Blood-Horse in December offered a great primer on the various "emergency" scenarios you might encounter in the foaling shed. The article was made available as a free download, and it's worth taking a look: When Foaling Goes Wrong. You never know when you're going to need this information -- now's a great time to read through the free report.