Sometimes planned meetings or calls don't always happen exactly according to schedule. Luckily, Activities in Pipedrive have an advanced 'marked as done' logic that will allow you to keep track of both when the activity was scheduled for as well as when the activity was actually marked as done.
The Calendar view will always show the activities by the scheduled due date, regardless of when they were marked as done.
The three activities below will still show as being scheduled to be completed on October 16, October 17, and October 18, even though they were all marked as done on October 16, prior to the due dates.
If you look at the activities in the detail views of the linked items, you will always see activities ordered by when they were actually marked as done (today, October 16). The three activities below show the done time in the description and are ordered by most recently 'marked as done' at the top.