Besides being a memorial for St. Robert Southwell, Martyr, it is also a memorial for St. Pietro Damiani, Doctor of the Church, and perhaps the most important theologian associated with the Gregorian Reform of the eleventh century. He is most famous today for his De divina omnipotentia, in which he considers what was historically called Jerome's Virgin and comes up with a controversial answer.
Jerome in one of his letters (Ad Eustochiam) advises a young woman to guard her virginity because it cannot be restored if lost, and he goes so far as to say that even God cannot restore a virgin. This letter came up in a discussion St. Peter Damian had with Benedictine monks at the monastery of Monte Cassino, and the abbot proposed a line of reasoning for saying that it was true, which required saying that God cannot do it because He does not will to do it. St. Peter wants to disagree with this. Obviously we do not want to go around saying that God can't do anything He doesn't will; that would mean, for instance, that God cannot make it rain today because He did not make it rain. This guts omnipotence of all force, and God ends up not being able to do anything that He does not actually do.
If we accept this, though, it introduces a complication in understanding Jerome's claim. St. Peter notes that obviously the claim cannot mean that God cannot undo any physical changes associated with loss of virginity; and likewise it cannot mean that God cannot by forgiveness and grace restored any purity associated with virginity. So the only thing the claim can mean is that God cannot make a non-virgin a virgin. Or on a more general level, since the asymmetry between virginity and nonvirginity is due to change through time: God cannot make the past not be the past. St. Peter denies that this is obviously true: God can, perhaps, make the past not be the past. Our assumption that God cannot (if we have that assumption) is like the abbot's idea that God cannot do something He does not.
Obviously it's the case that we must maintain the principle of noncontradiction. Damiani has occasionally been accused of throwing over noncontradiction, but this is quite clearly a misreading of his claims. But omnipotence requires that God omnia possit, can do all, in some sense of the term. We can make sense of saying that God can do all without also committing ourselves to the claim that God can do contradictions -- contradictions don't usually fall under 'all'. But what is the contradiction in claiming that God can make the past not to be the past? We can't do that, since we only have power over the future. But why can't divine omnipotence have power over the past as well as the future? We often talk about the necessity of the past -- but what is the actual necessity? We often say the past cannot be changed -- but by that we mean that our limited natural powers cannot do it, and what makes it so that unlimited divine power cannot?
Damiani's primary point in all of this is that when you are talking about omnipotence, you should never say "can't" lightly. If you cannot identify an actual contradiction, you don't have grounds for saying God can't do it. Going around saying that God can't do this, or that God can't do that, is a form of recklessness. As it happens, though, he thinks that in the case of Jerome's Virgin (and, more broadly, undoing the past) we can, in fact, identify a contradiction: it involves claiming that what God wills God also does not will. It's not the case that God can't do it because He does not will to do it; it's that it can't be done because it requires both willing and not willing it. (This is analogous to, but not quite the same as, what will be St. Anselm's slightly later and more widely popular response to the same problem, which argued for a distinction between antecedent and consequent necessity.)
You can read St. Peter Damian's De divina omnipotentia online in Spade's translation (PDF)
[ADDED LATER: Fixed a very confusing sentence that seemed to contradict everything else because it was originally in a different context.]
