If you go to the large and interactive version on PlanItMetro, you can mouse over individual squares to see the date as a tooltip.
The darkest red days have the lowest ridership, the darkest green the highest. You can see high ridership events like President Obama’s January
2005 2009 inauguration, the Stewart/Colbert rally in October 2010, Snowmageddon/Snowpocalypse in February 2010, and more.
Stepping back, it’s clear how ridership is highest in April, June, and July, and the number of very high ridership days jumped significantly in 2008 but then has stayed flat or a bit down since. Weekend ridership has gotten lower in recent years, probably because of all the trackwork.
What do you notice?