"Global Accessibility Awareness Day is a community-driven effort whose goal is to dedicate one day to raising the profile of and introducing the topic of digital (web, software, mobile app/device etc.) accessibility and people with different disabilities to the broadest audience possible."
Here's a few resources to start you on better understanding what accessibility is and why it's important.
- World Wide Web Consortium's (W3C) Accessibility information
- W3C Web Accessibility Initiative
- W3C Web Accessibility Content Guidelines overview
- Web AIM (Web Accessibility in Mind)