I solved my problem by changing "SchedulerWidget.js" in the Widget by following the steps below. (I am Japanese, so I changed "headerDateFormat" in Japanese.)
1. Change the file name from "mendix.SchedulerWidget.mpk" to "mendix.SchedulerWidget.zip".
2. Extract "mendix.SchedulerWidget.zip".
3. Open the "mendix>schedulerwidget" folder in the extracted folder.
4. Open "SchedulerWidget.js".
5. Search for "Sun_Mon_Tue_Wed_Thu_Fri_Sat" and replace it with "日曜日_月曜日_火曜日_水曜日_木曜日_金曜日_土曜日".
6. Close "SchedulerWidget.js.
7. Zip together the contents of the "mendix.SchedulerWidget" folder.
8. Rename this Zip file to "mendix.SchedulerWidget.mpk
9. Finally, add it to the widget folder of the project file and you are done.
Thank you for your clear answer and instructions!