From the Subscribe page in the Options dialog, you can manage your calendar subscriptions. Remind-Me can be subscribed to as many calendars as you'd like. Calendar subscriptions are URLs beginning with "webcal://". Each subscription is assigned a calendar name and URL. All events from a given subscribed calendar are placed in a category with the calendar name. Subscribed calendars can be automatically updated each day, or at any time you choose by going back to the Subscribe page and choosing the "Update Now" button.
Remind-Me supports being the default program to handle the webcal protocol. You can assign protocol to be handled by Remind-Me in Windows.
A good source of calendars you can subscribe to can be found at iCalShare.com.
In addition to "subscribing" to calendars, you can also import an iCalendar file on the Import page in the Options dialog.
Important Tip: When subscribing to a calendar do not also manually add events to this same category, as they will be replaced with the events from the subscribed calendar at the next update.