Sending Your Condolences for the Passing of Grand Duke Jean

HRH Grand Duke Jean of Luxembourg passed away on April 24th at age 98. If you would like, you can send a letter of condolence to his family.

You can find general info on writing to the Royals here. Along with some specifics on writing a letter of condolence here.

Who to write to: 
As Jean's wife passed away in 2005, the closest living relatives are Grand Duke Jean's children.
I would recommend sending your letter to Jean's eldest son, HRH Grand Duke Henri (Or TRH The Grand Duke & Grand Duchess.) In your letter, you can then mention you are sending your condolences to "you and your family."

But you can also address your letter to "The Family of Grand Duke Jean," or another member of the Royal Family if you wish.

The address: 

Palais Grand-Ducal
B.P. 331
L-2013 LUXEMBOURG

When to write: 
Don't feel like you need to write the minute you hear the news. Give yourself a few days to reflect, see what information is being released by the Palace, family, media, etc.

I think anytime within 2 weeks after the passing or shortly after the funeral is acceptable.