A possible date is the birthday of Kim Jong Il 16 February 1941, or the day before.
Indeed, that is currently the best bet. So if one of them goes off on his birthday, I wonder about the other one. Before or after the day? For dramatic effect, detonating one a few day before the birth day followed by more impressing one makes sense INHO. By "impressive," I mean the bigger yield or different design such as the one which can be mounted on a long range missile. The latter would effectively demonstrate the world that they basically one step away from acquiring ICBM.
However, there other factors are at play which we don't know much about. Political situation in Pyongyang and Kim's state of mind, along with Chinese pressure. How far would go to stop NK? What are they planning to do if NK refuses.