IRS Tax Calendar for Businesses and Self-Employed
You have the option to Subscribe to the Small Businesses calendar using Outlook 2007, Outlook 2010 or Mac iCal. Outlook 2003 does not have the ability to subscribe but you can download the tax events from the calendar. Downloading lets you add a one-time “snapshot” of the calendar events to your calendar program (e.g. Outlook).
Source: www.irs.gov