How implied vol varies vis-a-vis realised vol on an event days?
For a very short answer, given that the event is scheduled, the implied vol for a fixed future expiry date decreases, and the historical volality increases at event time. This could seem a bit counterintuitive but the implied vol factors in all scheduled forthoming events up to expiry. As the event has hit the market and its impact is priced into the underlying, there are less events left to expiry and less volatility to expect, so implied vol decreases. Historical vol on the other hand measures the impact from the event as long as the event stays in the historical measurement window, adding to the measure.
edit: Here is a link to a published research paper (Donders and Vorst: The impact of firm specific news on implied volatilities) giving you the longer answer with more details;