diff --git a/lib/schedule.rb b/lib/schedule.rb
index 3adda817..1dbd0962 100644
--- a/lib/schedule.rb
+++ b/lib/schedule.rb
@@ -11,7 +11,7 @@ module Schedule
   def prepare_schedule
     r = IceCube::Rule.send rule.split('_').first
     if rule == 'monthly_day'
-      r.day_of_week start_time.wday => [start_time.day / 7 + 1]
+      r.day_of_week start_time.wday => [(start_time.day - 1) / 7 + 1]
     else
       r
     end