diff --git a/src/Month.js b/src/Month.js
index 16a7ba786d..a8ebed04c4 100644
--- a/src/Month.js
+++ b/src/Month.js
@@ -178,7 +178,7 @@ export default class Month extends Component {
           {weeks.map(week => {
             let weekNumber;
             if (showWeekNumbers) {
-              weekNumber = DateUtils.getWeekNumber(week[0]);
+              weekNumber = DateUtils.getWeekNumber(week[6]);
             }
             return (
               <div
diff --git a/test/daypicker/events.js b/test/daypicker/events.js
index df70c797f4..92c7df4dd3 100644
--- a/test/daypicker/events.js
+++ b/test/daypicker/events.js
@@ -314,7 +314,7 @@ describe('DayPicker’s events handlers', () => {
       .find('.DayPicker-WeekNumber')
       .at(1)
       .simulate('click');
-    expect(handleWeekClick.mock.calls[0][0]).toBe(6);
+    expect(handleWeekClick.mock.calls[0][0]).toBe(7);
     expect(handleWeekClick.mock.calls[0][1]).toHaveLength(7);
   });
   it('should call `onWeekClick` when pressing ENTER on a week number', () => {
diff --git a/test/daypicker/rendering.js b/test/daypicker/rendering.js
index cefb83b091..7ae196c678 100644
--- a/test/daypicker/rendering.js
+++ b/test/daypicker/rendering.js
@@ -352,7 +352,7 @@ describe('DayPicker’s rendering', () => {
       <DayPicker showWeekNumbers initialMonth={new Date(2015, 1)} />
     );
     expect(wrapper.find('.DayPicker-WeekNumber')).toHaveLength(4);
-    expect(wrapper.find('.DayPicker-WeekNumber').at(1)).toHaveText('6');
+    expect(wrapper.find('.DayPicker-WeekNumber').at(1)).toHaveText('7');
   });
   it('should use the specified class names', () => {
     const wrapper = mount(