For details see http://globalaccessibilityawarenessday.org/: “On May 15, we invite you to participate in Global Accessibility Awareness Day (GAAD). The purpose of the day is to get people talking, thinking and learning about digital (web, software, mobile, etc.) accessibility and users with different disabilities.”
I have just a minor quibble with the above quote – I would have left “dis” off the last word. Electronic accessibility is really about optimizing the user experience for everyone.