You can create a link to the calendar from within the assessment and assignment properties, rather than from the calendar. When you create an assignment, under the due date there is the option to create a corresponding event in the Calendar.
In the properties, check the box labeled "Create a corresponding event in the Calendar tool."
An important difference to note between this feature in assignments and assessments: for an assignment, this will add an event to the calendar corresponding to the Due Date of the assignment (not the Cutoff Date). For an assessment, it will add an event to the calendar corresponding to the Start Time of the assessment. So, you may want to additionally add a manual entry when an assignment is made available (and/or an entry several weeks before the Due Date reminding students to work on it); and you may want to add a manual entry for the End Date of an assessment.
If the assessment or assignment is hidden from students, the event will not show up in the calendar.
