Do you mean after D-Day ?

Immediately after D-Day the allies built two Mulberry Harbours, here is a picture : http://www.war44.com/misc/images/5/WWII_Mulberry_harbour.jpg

The reason that they were needed is that the Allies had realised after the failure at Dieppe in 1942 that taking an already built harbour was too difficult. So, the invasion was based on beaches. This posed a problem in getting supplies to the invading troops. The Mulberry was the answer, ships could dock, be unloaded and then the cargo driven in trucks from the harbour to land.

The intention was that the harbours would last 3 months. The one on Omaha beach was destroyed in a storm on 19th June because it wasn't properly secured to the sea bed.

The one on Gold beach (later known as Port Winston) was secured to the sea bed and was in constant use for 6 months, at which point the Allies had taken Antwerp and switched supplies to there. If you look at the coastline at Arromanches some remains of the harbour can stiill be seen today, in fact at low tide you can walk out to the concrete cassions that made part of the breakwater.