on the happening of originary angst.
Dasein's foundational openness can be understood as an event. It is an event of opening, or the becoming manifest of the world. Recall that I suggested that Dasein itself can be understood and an event—the event of sense-making, which takes place in us. Originary angst would then be the inaugural and inaugurating episode of this event. It is the curtain rising on the stage, the floodlights illuminating the field, or the conductor raising her baton. This event is not only the first but that which makes all subsequent events possible. The openning of openness, or the becoming manifest of the world, is what first allows any happening of sense-making at all.
This event, of course, does not happen in time—or if it does, this is not what Heidegger is interested in. To call it the 'first' is to say that it is the most basic or most foundational. The event language is thus in a sense metaphorical since it implies a temporal ordering that does not apply here. But this metaphor is at least familiar: in philosophy, we often use temporal or dynamic metaphors to lay out relationships of ontological dependence and grounding. In any case, just as you can tell the story of the experience of angst in time, so too we can tell the story of the becoming-manifest of the world (the happening of originary ansgst) in the time of the a priori. This is the story of what it takes for there to be openness to being rather than not—an origin myth, if you will.
The key event in this story of the becoming manifest of the world is the sudden flaring up of ontological insight. As we have seen, the mood of angst provides ontological insight into what it takes to be an understander of being, specifically by revealing what it takes to be and to have a world. The original becoming manifest of the world is Desein's coming to have a world, or the world coming to be (because, recall, the world is not apart from its manifestness). Originary angst is thus Dasein's opening onto being—its opening to the fact that things are and to what they are. So there is not some particular ontological insight at stake here; it is the bursting forth of ontological insight per se: the understanding of being. This 'moment of clarity' is the granting of being, which Heidegger will later call 'Ereignis.'