You can predict the boost in earnings based on prior search trends. I will try to explain how I knew beforehand that there would be an increase in earnings for my online games website; although they weren't nearly as large as yours. In brief, I routinely use Google Trends (click here for a case in point) to get a gist of what the traffic will be like for the month. While this is an indicator of search trends and volumes, I assume it to be an overall snapshot of traffic. By looking at the trends for the term 'free online games', we can see an obvious increase in traffic every year around the month of March. As a result, it only makes sense that earnings should increase within this timeframe. I hope my answer somewhat explains why there may have been a partial boost in earnings, which I believe is far more imperative than by how much your earnings increased.
Last two days have been below average. Just finished tweaking 3 sites for 2 hours. Hopefully tomorrow will be above average. Keeping logs of your AdSense revenue and having charts comparing days gives you an amazing view at exactly what's going on. Revenue vs traffic, revenue vs different ad positions etc. I'm also trying to see if I can start making "Predictions" based on my stats and charts to motivate me more . Haven't made this available on my blog yet, but I might... not sure yet. EXCEL IS YOUR FRIEND! USE IT!