Riccardo,
I too had this same need and I could not figure out how to do it either. I ended up creating my own control. See a screenshot here: http://jjrock.com/VisualCalendar.jpg
It's ajax-ified but does not rely on client-side calendar javascript. I have also added functionality to set a selected date and provide a range of valid dates (minimum/maximum). It actually works pretty nice even though the code looks hacked together (because it was!)
I have been meaning to upload the control to the Downloads section but have not had time to clean and package it up. If you can hold on a bit I may be able to get to this sometime over the weekend.
Unless...somebody else has a better solution?
Thanks,
JJ