diff --git a/app/assets/stylesheets/application.css b/app/assets/stylesheets/application.css
index c013f9650..6efc9fac7 100644
--- a/app/assets/stylesheets/application.css
+++ b/app/assets/stylesheets/application.css
@@ -1245,12 +1245,14 @@ ul.cal {
 
 .cal .calbody p.day-num {font-size: 1.1em; text-align:right;}
 .cal .calbody .abbr-day {display:none}
-.cal .calbody.odd p.day-num {color: #bbb;}
+.cal .calbody.this-month {background-color:#fff;}
+.cal .calbody.other-month {background-color:#f6f7f8;}
+.cal .calbody.other-month p.day-num {color: #bbb;}
 .cal .calbody.today {background:#ffd;}
 .cal .calbody.today p.day-num {font-weight: bold;}
 
 .cal .calbody .icon {padding-top: 2px; padding-bottom: 3px;}
-.cal .calbody.nwday:not(.odd) {background-color:#f1f1f1;}
+.cal .calbody.nwday:not(.other-month) {background-color:#f1f1f1;}
 
 p.cal.legend span {display:flex;}
 .controller-calendars p.buttons {margin-top: unset;}
diff --git a/lib/redmine/helpers/calendar.rb b/lib/redmine/helpers/calendar.rb
index 9f3962236..05887ed34 100644
--- a/lib/redmine/helpers/calendar.rb
+++ b/lib/redmine/helpers/calendar.rb
@@ -57,7 +57,7 @@ module Redmine
       end
 
       def day_css_classes(day)
-        css = day.month==month ? +'even' : +'odd'
+        css = day.month==month ? +'this-month' : +'other-month'
         css << " today" if User.current.today == day
         css << " nwday" if non_working_week_days.include?(day.cwday)
         css