Complex sentence: Paid for by the Nazi Party, it was completed in 13 months and Hitler would make his first visit on September 16th, 1938, but held until a formal presentation to him on April 20,1939--a gift for his fiftieth birthday.
Split into simpler sentences: Paid for by the Nazi Party, it was completed in 13 months. Hitler first visited on September 16,1938, and returned on April 20,1939, for its formal presentation as a gift for his fiftieth birthday.