iCalendar is the standard Internet format for exchanging calendar information. It's supported by most calendar software including Google Calendar, Hotmail Calendar, Outlook, Apple iCal, and Lotus Notes.
When you use a standard calendar program to manage your web calendar, you solve two problems at once. The program can produce an HTML widget for display on your web page. It can also produce an iCalendar feed that enables that same information to appear automatically on other sites.
It's ironic but true that most web calendars are not produced by standard calendar programs. That means they're not available in iCalendar format. Instead they're available only as HTML, or sometimes PDF. People can view these calendars, but computers can't automatically exchange them and combine them.
If your calendar doesn't have an easy/obvious button or link to export iCal, then the answer is probably "it's not," but you should search your documentation (of your Web calendar) for "iCal" or "iCalendar" to make sure. You can also contact the vendor that makes your web calendar and ask them.
Some iCalendar-compliant programs can export an iCal file to your local computer, but can't automatically publish that file to the web as a "feed" available at a URL. To enable automatic transfer of your calendar you need to use a program that provides a feed. There are three general kinds of these:
"Cloud-based" calendar programs. Because programs like Google Calendar and Hotmail Calendar run "in the cloud" their iCal exports are automatically available as feeds. They also provide HTML widgets that embed in web pages. That makes these two programs ideal for publishing calendars that you write once, and make available as HTML for people, and as iCalendar for computers. Here's a tutorial on publishing iCalendar feeds using Google Calendar or Hotmail Calendar.
Desktop calendar programs. Some of these can "export to the web" -- notably Outlook and Apple iCal.
Content management systems. The WordPress blog publishing system, for example, can be outfitted with a plugin that enables you to write your calendar once, then publish as both HTML for people and iCalendar for computers. Unfortunately most CMSs that include calendar modules don't support iCalendar. If the calendar page produced by yours doesn't display an iCal link or button, check the documentation (or ask the vendor) if there's a way to turn that feature on. And if not, ask the vendor to add the feature! iCalendar has been an Internet standard for a dozen years. It should be supported.
The key benefits are:
Increased engagement. Visitors to your site can use their own personal calendar programs to subscribe to your iCalendar feed. If you're a school, for example, and you make your sports schedule available that way, a parent can see a child's soccer schedule alongside his or her own personal and business schedules.
Less work (and free advertising). Other sites can syndicate your iCalendar feed. If you're promoting a concert or a church supper, you'd like to type the information once for your site, and then have it appear automatically in other contexts. An iCalendar feed makes that possible. And if one or more other sites subscribe to your feed, it's automatic. You enter the event once, on your own calendar, and it propagates to those other sites.
No training costs. Many people already know how to use common iCalendar systems, like Google Calendar. They use them to manage their own personal and professional calendars. If you leverage that knowledge for the public calendar on your website there's nothing new to learn.
(from the elmcity faq)