I think that an easy way to customize an event, without having to persist the details per user, would be to use query parameters to customize the event. For example, if I wanted to create a countdown to the Anthem 5K Fitness Classic, a road race in Louisville, KY and the start of the Louisville Triple Crown of Running, I could specify the following URL:

http://days.to/until/7-march?description=Anthem%205K%20Fitness%20Classic

This would display the countdown as it does today, but would display the specified description as well. I am sure that other query parameters could be specified to expand the possibilities.

Show how many days left before an examination (or exam period/week)

On August 21st, there will be a total solar eclipse visible in the midwest portion of america.

Go to school or work dressed as the craziest thing you can be. Don't let the government tear down our fun!

September 19

Automatically post a countdown to a users facebook feed

National No school day!

9000

Allow users to track the numbers of days since certain life events such as starting a diet, stopping smoking, drinking or drugs.

An application programmers interface (API) to allow developers access to event data.

kids get the own day on jan.3

Allow users to select which country events are shown for. This will filter / only show events that apply to their country, and change dates/times to be specific to the selected country.

Internationalization / Localization.

HAPPY HUNGER GAMES....MAY the odds be ever in your favor.