When you create events in your org, they come with a start date and an end date. With the Event reminder package, you can send automated SMS few days/hours before the event start date to all the members associated with the event. All you need to do, is create an XML configuration with the Event reminder package, that would contain all the details of the SMS to be sent(mobile number, senderID, SMS template etc.).
Once the XML configuration is created, you just need to associate it with the workflow rule in the actions like this:
and make sure that you paste the XML configuration in double-quotes, like this:
Whenever the workflow rule would be triggered, an automated SMS would be sent out