If you have a repeating activity on your calendar, you have two options for deleting the activity, dependent upon whether you want to delete a single instance of the activity or the entire series.
As an example, let's say you have a daily repeating activity on your Calendar for the month of February 2017 - we'll call it Activity X:
Let's say you won't need to perform Activity X on February 13. Click Activity X on the 13th and you'll see the activity's Details page. On that page is an Actions box:
To delete just this instance of the Activity, you'll want to click "Delete Instance" here. This would leave all instances of Activity X on your Calendar, other than the instance scheduled for the 13th.
To delete the entire series, you can follow all of the steps above, except that you would need to click "Delete Series" to delete all of them.