It helps!

It is silly how I never noticed  the (W06) in top of agenda!

Anyway Nick comments

but I guess the more difficult question is a user interface that allows
you to construct such elaborate org-diary-class constructs.

are quite pertinent.

Daniel

2010/2/10 Carsten Dominik <carsten.dominik@gmail.com>

On Feb 10, 2010, at 11:20 PM, Nick Dokos wrote:

Daniel Martins <danielemc@gmail.com> wrote:

2010/2/10 John Rakestraw <lists@johnrakestraw.com>

Daniel Martins writes:

Someone could send me an example of the use of the org-diary-class ??

  *** 12:15-13:05 Class
     <%%(org-diary-class 1 13 2010 5 3 2010 1 3 7 10 14)>
     <%%(org-diary-class 1 13 2010 5 3 2010 3 10)>
     <%%(org-diary-class 1 13 2010 5 3 2010 5 10 13)>

  Class meets Mon-Wed-Fri from Jan 13 to May 3 from 12:15 to 13:05, with
  holidays as indicated. Friday's class, for example, doesn't meet in weeks
  10 and 13.

Thank you very much John. Incredibly fast answer !!!

BTW Is there an easy /practical  way to convert holidays dates to number of weeks?

Not sure I read this right, but if you are talking about converting a
date to a week-number (as e.g. exhibited at the top of the weekly
agenda), this should do the trick:

;;; date is a three-element list (month day year)
;;; (calendar-current-date) returns the date in this format.
(defun week-number (date)
     (org-days-to-iso-week
                  (calendar-absolute-from-gregorian date)))

but I guess the more difficult question is a user interface that allows
you to construct such elaborate org-diary-class constructs.

You can also navigate the agenda to the corresponding days, and it will show the week number.

HTH

- Carsten