Once again many thanks to Arturo Sparapano for this wonderful cover posted from Luxembourg!
The stamp commemorates former Grand Duke Jean's 90th Birthday, which took place on 5th Jan 2011.
Several times over the years, the Luxembourg P&T has released a stamp (or several) featuring a member of the Grand Ducal Family. Last year, stamps were released to mark Grand Duke Henri's tenth anniversary on the throne; and the 25th anniversary of Grand Duchess Charlotte's death in 1985. Stamps from the P&T from 1999 can be viewed on their website.