Go to this page: http://ipag.osug.fr/twiki/bin/view/Ipag/Intranet/Absences
Then go through the AGATE application (you will need your JANUS ID and passwd) and go to "Dépôt" to place your vacations, "Calendar" to have a general view of your absences, etc.
This works both for your holidays and for any type of absence.