Ovulating. As soon as you resume ovulating, so in theory the first month after giving birth. In practice that's pretty rare, and probably isn't such a great idea.
Any time. I tell patients that there are no guarantees and I have seen people get pregnant again in 4-6 weeks, but I personally tell them they should give time for the body to rest and allow a little gap between children. My own kids are 3 years apart and seemed like a perfect spread for us.