Once upon a time, researchers used periodical indexes to find articles discussing a particular person, business or subject. While most of that work is now done by searching databases, indexes are very useful for finding articles that are not available online.
The most common periodical index is the Reader's Guide to Periodical Literature, which indexes the most popular periodicals in the U.S. The Reader's Guide is available in most public libraries. In some public and special libraries you'll find The New York Times Index and/or The Wall Street Journal Index, which can help you find the date of a particular newsworthy even.
Other notable indexes include the Magazine Index, and the Alternative Press Index, which indexes alternative periodicals like the Village Voice and the Boston Phoenix.
In some places, the local library has created indexes for the local newspaper. For example, an index by the Enoch Pratt Free Library is the only practical way to locate older articles from the Baltimore Sun unless you have access to a database old Sun articles.
For related entries, search "News" as a Subject.