Derby Day falls on July 9 of each year, and commemorates 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 introduced on July 9, 2014.
2015EditWail of the Whale quest on the same day.
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.