I can't make the red alert on the calendar tab go away

The numbered red alert spot on the Calendar tab denotes an event that is scheduled for the current day. The number inside the spot will indicate how many events for which you are listed as an attendee.

Unlike the alert on the Messages tab, which goes away after your new messages are read, and the alert on the Tasks tab, which goes away after the tasks are checked off as being completed, this notification will remain whenever there is a calendar event scheduled on the current day.