starting_date = models.DateField(_('Дата начала'))
expiry_date = models.DateField(_('Дата окончания'))

from django import forms
from django.utils.translation import ugettext_lazy as _
from wonderay.main.models import Promotion

class PromotionAdminForm(forms.ModelForm):
    class Meta:
        model = Promotion

    def clean(self):
        self.cleaned_data = super(PromotionAdminForm, self).clean()
        if self.instance.starting_date > self.instance.expiry_date:
            raise forms.ValidationError(_('Дата начала акции должна быть меньше даты конца акции'))
        return self.cleaned_data

from django.contrib import admin
from wonderay.main import models, forms

class PromotionAdmin(admin.ModelAdmin):
    form = forms.PromotionAdminForm

admin.site.register(models.Promotion, PromotionAdmin)

class PromotionAdminForm(forms.ModelForm):
    class Meta:
        model = Promotion

    def __init__(self, *args, **kwargs):
        super(PromotionAdminForm, self).__init__(*args, **kwargs)
        if self.instance.discount == 1:
            self.fields['comment'].required = True