diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
index b2ea9552c..dc65edabd 100644
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -1095,9 +1095,11 @@ module ApplicationHelper
     unless @calendar_headers_tags_included
       @calendar_headers_tags_included = true
       content_for :header_tags do
-        # Monday, Sunday, Saturday
-        start_of_week = {1 => 1, 7 => 0, 6 => 6}[Setting.start_of_week.to_i] ||
-          l(:general_first_day_of_week, :default => '1') # use language
+        start_of_week = Setting.start_of_week
+        start_of_week = l(:general_first_day_of_week, :default => '1') if start_of_week.blank?
+        # Redmine uses 1..7 (monday..sunday) in settings and locales
+        # JQuery uses 0..6 (sunday..saturday), 7 needs to be changed to 0
+        start_of_week = start_of_week.to_i % 7
 
         tags = javascript_tag(
                    "var datepickerOptions={dateFormat: 'yy-mm-dd', firstDay: #{start_of_week}, " +