Quoting the GWX manual out of laziness:
-------
Campaign Start
The stock Silent Hunter III campaign began on September 1, 1939. Historically, U-boats began moving to their wartime patrol areas starting around August 15, 1939, so GWX has advanced the start of the campaign to August 1, 1939 to allow players to reach their historical patrol areas before the war begins.
--------
Most people do a short (4 or 5 days) shakedown patrol. Go sight seeing. 
Oh, btw, be sure to take a look at that GWX manual. It has tons of useful and interesting information! 
Benny