Why ‘at midnight’? That is the moment of least expectation. There is no thought of it. It is a moment of complete unawareness. It is as though one might calculate complacently, … ‘So many years have passed since Adam, and the six thousand years [since the beginning of Creation] are being completed, and then immediately, according to the computation of certain expositors, the day of judgment will come.’ Yet these calculations come and pass away, and still the coming of the bridegroom is delayed. So the virgins who had gone to meet him now are sleeping. But just when he is least looked for, when the best calculators are saying, ‘The six thousand years were waited for, and, look, they are already gone by. So how then shall we know when he will come?’—he comes at midnight. So what is ‘midnight’? It means when you are least aware.

- St Augustine of Hippo