Each unique coloured line is the event vol in %, 21 days before the event. The data is on Facebook. Each colour is a different earnings event.
As you can see most of the time the event vol is pretty steady. There is one time (aqua) where the event vol spiked going into the event. (Day21 is the day before the event, Day 1 is 21 days before the event).
Interesting chart.
That aqua line looks like it was the 30-Oct-18 earnings cycle, in which case the rise in the line can be explained by the general market volatility rise. VIX had gone up that month as the SPX started to wobble.
From my personal experience of trading straddles leading up to earnings, I have found that :
1) vega rarely makes up for the theta loss (this FB chart is a perfect example of an exception).
2) It's the movement in the underlying that gives the profit, and if this doesn't happen, then vol increase will just reduce your loss at best.
3) For most stocks the vol ramp happens in the last few days, and not weeks before the event. So, buying early means you are victim to the theta, but the volatility doesn't come to your rescue just yet.
4) The earlier you buy (say 4 weeks before event), the lower your delta and gamma, so the large the stock needs to move to make a profit.
5) Long straddles can be a nice little 'black swan' portfolio hedge. If you are lucky enough to have some of these trades open at the time when the SPX takes a dive (Feb-18, Dec-18 etc), then you are sitting on very decent profits as the gamma and increased vega both echo the 'Ker-ching' sound in the background.
Happy trading.