From the page I quoted:
{G}uided tours in English are held every Wednesday, Saturday and Sunday : 11:30am / 2:30pm
SUMMER SCHEDULES from July 1 at August 31, 2008, every day : 11:30am / 2:30pm
To me, every day means every day - French people are quite particular about describing their operating hours and days.
As
iff mentioned, however, there is a phone number listed on the site I linked to that should be able to give you more information. Skype is often plugged on FT as a cheap way to call internationally; calling the ONP would be USD .021/min and you would dial +33141100810 from Skype to do so.
Peace.