Derby Day was a past celebration that fell on July 9 of 2014 and 2015. It commemorated the day Habitica defeated the worst of its ancient bugs - DerbyJS.
When first introduced, Habitica used a framework known as DerbyJS. The website had various issues with slowness, stability, and bugs in those early versions, and many users abandoned their characters because of those difficulties. After switching frameworks to AngularJS, Habitica's stability improved tremendously. To honor the switch, Derby Day was celebrated on July 9th of 2014 and 2015.
As part of the 2014 Derby Day celebration, each user received one free seahorse egg. Habitica also released The Dilatory Derby quest on the same day.