And yet, there are many times in which you may find yourself estimating probabilities on things that have already happened. Whether an event happened in the past or will happen in the future doesn’t make too much difference, it’s the knowledge you have about an event that defines its probability (for you). But the spirit of this question is really about some kind of “objective probability”. Surely everything in the past either happened or didn’t, it’s just a matter of finding it out. The past genuinely is in multiple-states, and as a result the probabilities of events in the past can be damn near anything.