Open PowerPoint and click the "LiveSlides" tab on the ribbon in the upper right.
Next click the "Live slide" button to bring up the LiveSlides dialog.
Go to the Google Calendar website.
You'll need to make your calendar public by following these directions.
Copy the URL from your browser.
Paste the URL into the LiveSlides dialog and click "Insert".
A placeholder image will now appear in your slide.
Enter presentation mode to view the live web content.
Select your presentation software for instructions on inserting Google Calendar into Keynote or PowerPoint on Mac.