The Calendar is a personal tool that displays all the events in all your enrolled courses including start dates, end dates, due dates and calendar events. Assignment folder dates automatically show in the Calendar when created. Please note that when you copy assignment folders from another course or a previous instance of your course, you will need to reset the dates of the folders or they will not show up in the Calendar.
Create and Manage Tasks in the Calendar: https://community.d2l.com/brightspace/kb/articles/18127-manage-course-events-with-the-calendar-tool
Creating calendar events: