Basic Points

• Event Vol is an abbreviation meaning "event IV". Regarding the core meaning of IV (implied volatility), it is the expected percentage range that the options market is pricing in as a one standard deviation move (68.3% chance) over the next year. 
• In contrast, event vol is how IV tends to increase significantly beyond its averages on days scheduled to have big events for that security–or the market as a whole. 
• In other words, event vol is a swelling of options premium (measured in excess IV%) which is tied to a significant market catalyst such as earnings, economic calendar events like CPI or interest rate announcements, or geopolitical events like major elections. 
• As there is heightened uncertainty around these events, options will predictably trade at a higher-than-average prices. This is the market pricing in larger stock movements. The Founder’s Notes make a point each day to warn of major economic events in advance right at the top.

Intermediate: Quantitative Technique

When modeling realized volatility (the expected movement with 68.3% confidence based on historical data) for the sake of a forecast, days with known event vol are often omitted from the time series so as not to overinflate forecasted volatility. Otherwise, one of the problems that arises is windowing which is how there would be a deceptive and abrupt shift in a moving average after event vol enters or exits a lookback window (the elements being averaged). 

Advanced: Strategy

Traders know that very often event vol gets removed from the market after an event, which makes for profitable premium selling before the events. However, this comes with a significant amount of tail risk (unlikely but devastating outcomes) when approaching this with short options. Therefore, it is often prudent for the sake of ergodicity (profiting from averages while avoiding suffering from fringe possibilities) to limit size and perhaps to set stop-limits in place for emergency and automated delta-hedging.