How to format (succinctly) a date when returning django validation?

In django, I'm validating a date:

from datetime import date
from django.utils.translation import ugettext_lazy as _
from django.core.exceptions import ValidationError

if start_date < date.today():
    raise ValidationError(
        _('%(start_date)s must be on or after today.'),
        params={'start_date': start_date},
    )

where start_date is a datetime instance.

What do I need to write in place of s in %(start_date)s in order to format the date nicely; preferably as dd-mmm-yyyy?

from How to format (succinctly) a date when returning django validation?