You have the option to either
the IRS Tax Calendar for Businesses and Self-Employed.
Subscribing adds web calendar data to your calendar program.
The web calendar data will be updated automatically, but you will not be able to make manual changes to it.
Downloading lets you add a one-time "snapshot" of the calendar events to your calendar program (e.g. Outlook.)
Your calendar program will allow you to edit this imported calendar data, but will not automatically refresh it when data is updated by IRS.