Hi Ramla,

To create an onchange function, in your <models>.py file add

@api.onchange('date_fin')
def _method_name(self):    
    if self.date_fin:
        if self.date_fin == date.today():    #assuming that date is a datetime object
            self.statut = 'desactiver' 

And to make the fields set to readonly in your xml file, i suggest you use 

<field name="field_name" attrs="{'readonly':[('statut', '=', 'desactiver')]} />

Just work around on this and I'm sure you'll get what you want.. 

Hope it helps. :)