I tried Erel's code which is similar to the ones I tried myself, however, none of them work.
What I tried in some cases just call "com.android.calendar" using an intent, which works, but some phones like HTC for example have their own version of calendar "com.htc.calendar", that's why I was asking if there was a "generic" intent to open the default calendar app.
It seems is not possible.
Thanks for your help folks.