Fix first day of week in dob customer field

Author: joachimVT

app/code/Magento/Customer/Block/Widget/Dob.php

@@ -186,7 +186,8 @@ public function getFieldHtml()
             'max_date' => '-1d',
             'change_month' => 'true',
             'change_year' => 'true',
-            'show_on' => 'both'
+            'show_on' => 'both',
+            'first_day' => $this->getFirstDay()
         ]);
         return $this->dateElement->getHtml();
     }
@@ -307,4 +308,17 @@ public function getMaxDateRange()
         }
         return null;
     }
+
+    /**
+     * Return first day of the week
+     *
+     * @return int
+     */
+    public function getFirstDay()
+    {
+        return (int)$this->_scopeConfig->getValue(
+            'general/locale/firstday',
+            \Magento\Store\Model\ScopeInterface::SCOPE_STORE
+        );
+    }
 }

lib/internal/Magento/Framework/View/Element/Html/Date.php

@@ -26,6 +26,7 @@ protected function _toHtml()
         $changeYear = $this->getChangeYear();
         $maxDate = $this->getMaxDate();
         $showOn = $this->getShowOn();
+        $firstDay = $this->getFirstDay();
 
         $html .= '<script type="text/javascript">
             require(["jquery", "mage/calendar"], function($){
@@ -59,6 +60,7 @@ protected function _toHtml()
             ($changeMonth === null ? '' : ', changeMonth: ' . $changeMonth) .
             ($changeYear === null ? '' : ', changeYear: ' . $changeYear) .
             ($showOn ? ', showOn: "' . $showOn . '"' : '') .
+            ($firstDay ? ', firstDay: ' . $firstDay : '') .
             '})
             });
             </script>';