I guess one of them will be HEU. N. Korea has large uranium deposits. I heard experts saying that it is low grade and hard to enrich it to weapon's grade. On the other hand, they can locally produce bomb material unlike limited and dwindling supply of plutonium. So they mine as much uranium ore as possible, and run vast number of centrifuges. That will provide enough uranium for them. They have proven to be more determined in this regard than outsiders expected. There is even speculation about 'boosted nuke bomb' which is said to be somewhere between fission nuke and fusion nuke(hydrogen bomb.) Nobody knows before they show it, though.
If they do nuke test in the tunnels which are in close proximity to each other and do it in short succession, I wonder if it can damage underground geological structure in some way, resulting in some unexpected disaster.
A possible date is the birthday of Kim Jong Il 16 February 1941, or the day before.