This year's special is a exhibit called La Dynastie à Travers les Timbres, roughly translated 'The Dynasty Through Stamps.' The exhibit mainly concentrates on stamps of the head of states starting from Grand Duke Adolphe until today, but don't worry, it also contains stamps of Grand Duke Jean's siblings and children as well as Henri's children.
The awesome Manuel Dias was among the pre-visitors to the exhibit; a gallery giving a glimpse of the exhibit is available on his website.
So should you be in or around the Grand Duchy this summer, I highly recommend to visit the palace. To see it from the inside is worth the trip alone, the stamp exhibit is only the icing on the cake.
For more information please refer to the Cour Grand-Ducale, Wort, Tageblatt and the LCTO.