Grey Literature Day is commemorated on May 16th
Although it may not be considered strictly scholarly, grey literature is an important source of information because it is produced by researchers and practitioners in government agencies, universities, corporations, research centers, associations and societies, and professional organizations.
"Grey Literature is a field in library and Information science that deals with the production, distribution, and access to multiple document types produced on all levels of government, academics, business, and organization in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body." (Greynet.org)