models.py

class Event(models.Model):
    name = models.CharField(max_length=500)
    start_date = models.DateField(default=datetime.date.today() + datetime.timedelta(days=1))
    end_date = models.DateField(blank=True, null=True)
    start_time = models.TimeField(blank=True, null=True)
    end_time = models.TimeField(blank=True, null=True)
    description = models.TextField('Full event description', blank=True)
    website = models.CharField(max_length=500, blank=True)
    map_code = models.CharField(max_length=500, blank=True)
    publish_status = models.CharField(max_length=15, choices=PUBLISH_STATUS, default=UNSAVED)

    UNSAVED = 'unsaved'
    PREVIEW = 'preview'
    DRAFT = 'draft'
    PUBLISHED = 'published'

    PUBLISH_STATUS = (
        (UNSAVED, 'Unsaved'),
        (PREVIEW, 'Preview'),
        (DRAFT, 'Draft'),
        (PUBLISHED, 'Published'),
    )

    def save(self, *args, **kwargs):
        if self.publish_status == self.UNSAVED:
            self.publish_status = self.DRAFT
        super(Event, self).save(*args, **kwargs)