diff --git a/apps/payment/models.py b/apps/payment/models.py
index 2de1c26a..a55ce8c5 100644
--- a/apps/payment/models.py
+++ b/apps/payment/models.py
@@ -9,6 +9,8 @@ from django.contrib.postgres.fields import ArrayField, JSONField
 from django.core.validators import RegexValidator
 from django.utils.timezone import now
 
+from project.utils import weekday_in_date_range
+
 from apps.course.models import Course
 from apps.config.models import Config
 from apps.school.models import SchoolSchedule
@@ -164,7 +166,11 @@ class SchoolPayment(Payment):
         ).aggregate(
             models.Sum('month_price'),
         )
-        month_price_sum = aggregate.get('month_price__sum', 0)
+        if self.add_days:
+            weekday_count = weekday_in_date_range(self.date_start, self.date_end, self.weekdays[0])
+            month_price_sum = aggregate.get('month_price__sum', 0) * weekday_count / 30
+        else:
+            month_price_sum = aggregate.get('month_price__sum', 0)
         if month_price_sum > config.SERVICE_DISCOUNT_MIN_AMOUNT:
             discount = config.SERVICE_DISCOUNT
         else:
diff --git a/project/utils.py b/project/utils.py
new file mode 100644
index 00000000..38f04b76
--- /dev/null
+++ b/project/utils.py
@@ -0,0 +1,14 @@
+from datetime import timedelta
+from collections import Counter
+
+
+def date_range(start, end):
+    delta = end - start
+    for d in range(delta.days + 1):
+        yield start + timedelta(days=d)
+    return
+
+
+def weekday_in_date_range(start, end, weekday):
+    counter = Counter([d.isoweekday() for d in date_range(start, end)])
+    return counter.get(weekday, None)