There are two great times to visit Germany, 1. just before Christmas and 2. during the summer.
Just Before Christmas :
During Christmas Germany comes alive with Christmas markets and lights hanging from all buildings. The Christmas markets are outdoors so be prepared for the cold and try warming up with some gluehwein (glue vine). Gluehwein is fruity wine served warm; think a mix between hot apple cider and wine.
Be careful gluehwein can go straight to your head. The winter isn't a huge tourist season so you will have the benefit of smaller crowds and mixing more with the locals. However, there will also be less tour times at all the tourist destinations such as castles.
If you are traveling with a significant other, winter is a very romantic time. Remember to book your ticket early as they can get expensive the closer Christmas gets.
During the summertime Germany's landscape is a beautiful bright green, purple and red from the blooming flowers and freshly alive grass. The wine regions are also alive with ripening grapevines. Everyone leaves their house and sits at outdoor cafes as much as possible.
Germans also love going for hikes in the near by woods and mountains. The summertime is also great for sight seeing as everything is in full bloom around the beautiful old castles and tour hours are extended.
However, most people visit during the summer so you might have to fight the crowds. Flights to Germany are also expensive during the summertime so book early.