Ensure weekday labels respect the locale’s firstDayOfWeek

src/Month.js

@@ -29,6 +29,7 @@ export default function Month({
     <div className={ className }>
       {React.cloneElement(captionElement, captionProps)}
       <Weekdays
+        firstDayOfWeek={ firstDayOfWeek }
         locale={ locale }
         localeUtils={ localeUtils }
         weekdayComponent={ weekdayComponent }

src/Weekdays.js

@@ -2,17 +2,19 @@ import React, { PropTypes } from 'react';
 import DayPickerPropTypes from './PropTypes';
 
 export default function Weekdays({
+  firstDayOfWeek,
   locale,
   localeUtils,
   weekdayComponent,
   weekdayElement,
 }) {
   const days = [];
   for (let i = 0; i < 7; i += 1) {
+    const weekday = (i + firstDayOfWeek) % 7;
     const elementProps = {
       key: i,
       className: 'DayPicker-Weekday',
-      weekday: i,
+      weekday,
       localeUtils,
       locale,
     };
@@ -32,6 +34,7 @@ export default function Weekdays({
 }
 
 Weekdays.propTypes = {
+  firstDayOfWeek: PropTypes.number.isRequired,
   locale: PropTypes.string.isRequired,
   localeUtils: DayPickerPropTypes.localeUtils.isRequired,
   weekdayComponent: PropTypes.func,

test/DayPicker.js

@@ -7,6 +7,7 @@ import { shallow, mount, render } from 'enzyme';
 import { expect } from 'chai';
 import sinon, { spy } from 'sinon';
 import DayPicker from '../src/DayPicker';
+import * as LocaleUtils from '../src/LocaleUtils';
 import keys from '../src/keys';
 
 describe('<DayPicker />', () => {
@@ -159,6 +160,11 @@ describe('<DayPicker />', () => {
       );
       expect(wrapper.find('.DayPicker-Day')).to.have.length(42);
     });
+    it('should render weekday labels accounting for locale settings', () => {
+      const localeUtils = Object.assign({}, LocaleUtils, { getFirstDayOfWeek: () => 1 });
+      const wrapper = mount(<DayPicker localeUtils={ localeUtils } />);
+      expect(wrapper.find('.DayPicker-Weekday').at(0)).to.have.text('Mo');
+    });
   });
 
   describe('day modifiers', () => {