1) Anniversary 2) Day of remembrance 3) Semicentenary 4) The 50th anniversary 1) Semicentenary 
• (a.) Of or pertaining to half of a century, or a period of fifty years; as, a semicentennial commemoration. • (n.) A fiftieth anniversary.
Sem`i·cen·ten'ni·al adjective Of or pertaining to half of a century, or a period of fifty years; as, a semicentennial commemoration. 
semicentenary adjective of or relating to or marking the 50th anniversary