I have been asked the question of how to do this a few times. Finally got five minutes to research it.
The answer is quite clever and simple. It can be done using standard SharePoint and Outlook features.
- Create a view showing items in the calendar with a start time of Today
- Connect to the RSS feed from the SharePoint view using Outlook
- Use rules and alerts in Outlook, to set up an Alert on new items in the RSS feed
Steps in Detail
There are two good articles that outline the process in detail
- Getting SharePoint Calendar Reminders in Outlook by Paul Liebrand
- How to setup Outlook reminder for SharePoint calender events by Rohan Chittyal