def translate(english_days):
    """ translate the name of a day in a week from English to French.

    :param english_days: string, a day in a week in English.
    :return: string, the French name for the day.
    """
    french_days = ''
    if english_days == 'Monday':
        french_days = 'lundi'
    elif english_days == 'Tuesday':
        french_days = 'mardi'
    elif english_days == 'Wednesday':
        french_days = 'mercredi'
    elif english_days == 'Thursday':
        french_days = 'jeudi'
    elif english_days == 'Friday':
        french_days = 'vendredi'
    elif english_days == 'Saturday':
        french_days = 'samedi'
    elif english_days == 'Sunday':
        french_days = 'dimanche'
    else:
        raise Exception("Days in wrong form!")
    return french_days