Примерно так будет работать.
#models.py
DAYS_OF_WEEK = (
(0, 'Monday'),
(1, 'Tuesday'),
(2, 'Wednesday'),
(3, 'Thursday'),
(4, 'Friday'),
(5, 'Saturday'),
(6, 'Sunday'),
)
days = models.CharField(max_length=1, choices=DAYS_OF_WEEK
#forms.py
widgets = { 'days': forms.CheckboxSelectMultiple }
Или сохранить несколько дней
#models.py
class Days(models.Model):
day = models.CharField(max_length=8)
days = models.ManyToManyField(Days)
#forms.py
widgets = { 'days': forms.CheckboxSelectMultiple }