I mentioned before that I intend writing something about the notion of the 'Event'. By way of preparation, here is Zizek explicatiing Badiou's notion with reference to the French Revolution:
Take, for example, French society in the late eighteenth century: what is accessible to Knowledge is the state of society-its strata, economic, political, and ideological fights, and so on; no Knowledge, however, enables us to predict or account for the properly unaccountable Event which consists in the so-called French Revolution. In this precise sense, an Event emerges ex nihilo. But the fact that it cannot be accounted for in the terms of the situation does not mean that it is simply an intervention from Outside or Beyond: it attaches itself, precisely, to the Void of every situation-to its inherent inconsistency and/or its Excess. The Event is the Truth of the situation, that which renders visible/readable what the "official" state of the situation had to "repress," but it is also always localized, that is, the Truth is always the Truth of a specific situation. The French Revolution is the Event that renders visible/readable the excesses and inconsistencies, the "lie," of the ancien regime; and it is the truth of the ancien regime situation-what is localized, or attaches to it. An Event thus involves its own series of determinations: the Event itself; its denomination ("French Revolution" not being an objective-categorizing designation but part of the Event itself, the way its participants or adherents perceived and symbolized their activity); its ultimate Goal (the society of fully realized emancipation, of freedom-equality-fraternity); its "operator" (the political movements struggling for the Revolution); and, last but not least, its subject (the agent who, on behalf of the Truth-Event, intervenes in the historical multiple of the situation and discerns/identifies in it the signs-effects of the Event). What defines the subject is his fidelity to the Event: coming after the Event, the subject persists in discerning its traces within the situation. "Subject," for Badiou, is thus a finite contingent emergence: not only is Truth not "subjective" in the sense of being subordinated to the subject's whims, but the subject himself is "serving the Truth" which transcends him; since he is never fully adequate to the infinite order of Truth, the subject always has to operate within a finite multiple of a situation in which he discerns the signs of Truth