Via France, it looks like, with a single sheet inside.

Contents:

envelope
stamps and postmark
letter

Beacon:

RE postmark:
Ecole Militare is the Military School. As far as I can remember it is beside Les 
Invalides (an invalide hospital set up for war veterans).
 
RE letter contents:
As regards dates being important. One wonders which dates. those on the letter, those 
on some letters or those on all ADW adverts? Maybe it is only dates gained from the 
Biblical references converted by excel into dates. Or only Julian dates? As
usual ambiguity possibly leads to more possibilities for speculation than before 
receipt of the letter.

The letter suggests to me that the dates given in the announcements may represent only an ordering of events and may not
directly relate to the timescale indicated ie. that the events may take place over a longer period and that it is the order in
which they occur that is of most importance.  I don't know if this is what they're picking up on, but I did post a couple of things
that were particularly concerned with the dates given on the most recent announcement (5/1/03) - this concerned the dates of
Trinity Sunday, Whitsun etc and the way that the agenda related to them.

Pierre Simon Laplace - whose quote on probability shows up a couple of times in the ads - attended the École
Militaire. I was just trying to find out more about  Laplace here:
http://physics.hallym.ac.kr/reference/physicist/Laplace.html - "At the age of 19, mainly through the influence of d'Alembert,
Laplace was appointed to a chair of mathematics at the École Militaire in Paris on the recommendation of d'Alembert. 

Correction: I meant to say that Laplace TAUGHT at the Ecole Militaire.

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