Is the Second Coming a contingent event, or is its day and hour fixed?

No, it is not contingent; there is one day and one hour, God knows it, and it will not change. Think about it logically: since it happens only once, there cannot be several possible days and hours on which Christ could return; there can be only one coming, so there is nothing contingent about it. Scripture speaks of the day and the hour as a single, solitary event of which there is no other, so it makes no contingencies. There may be other events surrounding it that are contingent, but not the day and the hour of his return.