Yes you can. By default Salesforce does not allow you to trigger an Email Alert based on Activity Conditions. So here is what we did to get around this. Go to your Lead, Opportunity, Account or Contact Object and either go to "Fields" or "Page Layout". You'll notice we've added the following fields to that Object;


Event Fields

  1. Next Meeting
  2. Meeting Subject
  3. Meeting Details
  4. Meeting Location
  5. Minutes Before Meeting
  6. Hours Before Meeting
  7. Days Before Meeting


Task Fields

  1. Activity Date    
  2. Activity Description
  3. Activity Status    
  4. Activity Subject
  5. Activity Type    


These fields are automatically updated any time an Event is Created or a Task is Completed. (You should test this functionality on your records, by creating a completed task and a new event, just to see what gets updated)


Because of these new fields we've added, you can build an Email Alert based on these feilds. Keeping in mind though. These fields will only store the LAST Completed Task and the Next Scheduled Event. 

NOTE: When setting up your Workflow Rule do not select "Tasks" as the object you are basing your Rule on. Select the Contact or Opportunity, or Lead, etc ...