A Lincoln Day Dinner is an annual celebration of the Republican Party and a fundraising event for Republican Party affiliated organizations at the county level. It is an opportunity for Republicans to gather together, network, and hear directly from officer holders and/or other influential speakers.
The event is named after Abraham Lincoln, the first elected president of the Republican Party who helped found and shape the party, and is famous for having issued the Emancipation Proclamation which freed all slaves.
Subsequent to the election of Ronald Reagan (1980), the most popular Republican president since Lincoln, some counties renamed the dinner after Reagan or have added his name to it, resulting in the names Reagan Dinner, Reagan Day Dinner, Lincoln–Reagan Dinner, etc.
Check out the links for recent Lincoln-Reagan DAy Dinners hosted by the AAGOP.