Legal opinion about invocations from The Humanist Society

http://thehumanistsociety.org/wp-content/uploads/2015/10/Secular-Invocation-Memorandum-5-5-14.pdf

Quote from above link:

Legal Memorandum: Secular Invocations The constitutional requirements governing legislative prayers require local government entities to authorize non-theistic invocations whenever theistic invocations are authorized.