The trouble with mares and "due dates" is that they don't really have a set due date :P typical gestation is 340-370 days. Foals are viable anywhere from 320 days, and I think there was a pregnancy that went over 400. Minis usually have a slightly shorter gestation, from what I know, but they still have a pretty broad window of typicality.
You will need to watch for a red bag delivery, minis are more prone to it than others and if the foal is red bag it requires immediate intervention via cutting of the bag (I have heard of using a popstick cut diagonally) because red bag is when the placenta comes out first and the foal will need to be able to breathe without waiting for its mother to pass enough of the bag for it to break through the weaker white part. The foal will also need a vet asap in the event of a red bag for a plasma transfusion.
That's all from my research of course :)