I test your script.

I get an alert "false".

How do i "put" my data (for example "c.Date__c" , and where, in the controller and in the VF page ???

Please use below code:

Simplified Controller:

public with sharing class recup_donnees_test_Controller {
    
   public LIST<Calendrier__c> Calendriers_Interne = new LIST<Calendrier__c>{get;private set;}
   public String ErrorOcc{get;private set;}
    Public recup_donnees_test_Controller (){
        
        Calendriers_Interne = [Select nom__c, id__c, Tache__c, SystemModstamp, Ressource__c, OwnerId, Name, LastModifiedDate, LastModifiedById, LastActivityDate, IsDeleted, Id, Heure_Fin__c, Heure_Debut__c, Date__c, CreatedDate, CreatedById, Categorie__c From Calendrier__c WHERE Ressource__c = 'FRANCO-IDRISS Béatrice' limit 1];
 
      for(Calendriers_Interne tempCal :Calendriers_Interne)
      {      
           Errorocc=tempCal.Date__c;
      }


    }
       

}

Visual Force Page

<apex:page Controller="recup_donnees_test_Controller" >
<script>
var ErrorOcc='{!ErrorOcc}'; 
alert(ErrorOcc);
</script>
    <apex:pageBlock title="Calendrier">
    <apex:pageBlockTable value="{!Calendriers}" var="c">
        <apex:column headerValue="nom" value="{!c.nom__c}"/>
        <apex:column headerValue="Tâche" value="{!c.Tache__c}"/>
        <apex:column headerValue="Ressource" value="{!c.Ressource__c}"/>
        <apex:column headerValue="Date" value="{!c.Date__c}"/>
        <apex:column headerValue="Heure Début" value="{!c.Heure_Debut__c}"/>
        <apex:column headerValue="Heure Fin" value="{!c.Heure_Fin__c}"/>
 
    </apex:pageBlockTable>
</apex:pageBlock>

i get en error :

Erreur : recup_donnees_test_Controller Erreur de compilation : expecting right curly bracket, found ';' à la ligne 3 colonne 79

but it seems to be well wrote ( i copy the script and your script seems to be ok). So i don't find the cause of the error...

I understand that you are helping me!

Replace 
public LIST<Calendrier__c> Calendriers_Interne = new LIST<Calendrier__c>{get;private set;}

with 
public LIST<Calendrier__c> Calendriers_Interne{get;private set;}

 and initialize it in constructor as 

Calendriers_Interne = new LIST<Calendrier__c>();

I get an error:

Erreur : recup_donnees_test_Controller Erreur de compilation : unexpected token: '=' à la ligne 4 colonne 23

Now my controller is:

public with sharing class recup_donnees_test_Controller {
   
   public List<Calendrier__c> Calendriers_Interne{get; private set;}
   Calendriers_Interne = new LIST<Calendrier__c>();
   public String ErrorOcc{get;private set;}
    Public recup_donnees_test_Controller (){
       
        Calendriers_Interne = [Select nom__c, id__c, Tache__c, SystemModstamp, Ressource__c, OwnerId, Name, LastModifiedDate, LastModifiedById, LastActivityDate, IsDeleted, Id, Heure_Fin__c, Heure_Debut__c, Date__c, CreatedDate, CreatedById, Categorie__c From Calendrier__c WHERE Ressource__c = 'FRANCO-IDRISS Béatrice' limit 1];
 
      for(Calendriers_Interne tempCal :Calendriers_Interne)
      {     
           Errorocc=tempCal.Date__c;
      }


    }
      

}

I asked you to initialize list in constrctor so your code should be like this

public with sharing class recup_donnees_test_Controller {
   
   public List<Calendrier__c> Calendriers_Interne{get; private set;}
   
   public String ErrorOcc{get;private set;}
    Public recup_donnees_test_Controller (){
        Calendriers_Interne = new LIST<Calendrier__c>();
        Calendriers_Interne = [Select nom__c, id__c, Tache__c, SystemModstamp, Ressource__c, OwnerId, Name, LastModifiedDate, LastModifiedById, LastActivityDate, IsDeleted, Id, Heure_Fin__c, Heure_Debut__c, Date__c, CreatedDate, CreatedById, Categorie__c From Calendrier__c WHERE Ressource__c = 'FRANCO-IDRISS Béatrice' limit 1];
 
      for(Calendriers_Interne tempCal :Calendriers_Interne)
      {     
           Errorocc=tempCal.Date__c;
      }


    }
      

}

with your the new script, i get an error again...:

Erreur : recup_donnees_test_Controller Erreur de compilation : Invalid type: Calendriers_Interne à la ligne 10 colonne 11

oh ok I forgot that date__c is date type so replace 

public String ErrorOcc{get;private set;}

to 

public Date ErrorOcc{get;private set;}

error again...

Erreur : recup_donnees_test_Controller Erreur de compilation : Invalid type: Calendriers_Interne à la ligne 11 colonne 11

The 'date__c' is a custom field, so it gives data. I doubt that change it for 'date' could be a good thing...

public with sharing class recup_donnees_test_Controller {
  
   public List<Calendrier__c> Calendriers_Interne{get; private set;}
  
   public Date ErrorOcc{get;private set;}
  
    Public recup_donnees_test_Controller (){
        Calendriers_Interne = new LIST<Calendrier__c>();
        Calendriers_Interne = [Select nom__c, id__c, Tache__c, SystemModstamp, Ressource__c, OwnerId, Name, LastModifiedDate, LastModifiedById, LastActivityDate, IsDeleted, Id, Heure_Fin__c, Heure_Debut__c, Date__c, CreatedDate, CreatedById, Categorie__c From Calendrier__c WHERE Ressource__c = 'FRANCO-IDRISS Béatrice' limit 1];
 
      for(Calendriers_Interne tempCal :Calendriers_Interne)
      {    
           Errorocc=tempCal.Date__c;
      }


    }
     

}

Change datatype to again String and replace 

    Errorocc=tempCal.Date__c;   

with

  Errorocc= String.valueOf(tempCal.Date__c);