After many months of debating whether or not to invade Norway,  Hitler fear Britain occupying Norway first got the best of him.  Hitler decided that his only chance of defeating a better naval power was the advantage of a suprise attack.  On March 4th HItler cancelled all other naval operations and held all of German's ships in their port.  He then dispatched eleven long range submarines to main ports in Norway.  He planned a large scale invasion on April 9th, naming it Weser Day, setting Weser time at 05:15.
    Germany was not the only one planning operations in Norway.  The allies had devised their own plan, Wilfred, to control Narvik and the Swedish boarder.  Plan R4 was to lay two minefields in Norwegian waters, the first at Vestfjord and the second between Ålesund and Bergen.  Maybe laying a third at Molde.  The Royal Marines began the operation on April 8th, but knowledge of the German invasion forced them to cancel all plans and go on the defensive.  A British fleet was sent to sea to intercept the German naval force.
 

Map of Germany's plan of attack.  Six different battle groups where to attack six prime areas of Norway.

Map from http://home.sol.no/~kurtmon/weserubung2.html