Blood can be donated at Vereinshaus in Bad Homburg-Gonzenheim (Am Kitzenhof 4) on Monday, 20th February, 2012 between 5pm and 8.30pm.
To donate you must be healthy and between 18 and 68 years of age. If you have not donated before, then you must not be over 60. You must take some form of identification with you, such as a passport, that contains a photo.
You can read more about what then happens here (in German).