The holiday season is the biggest time of year for Americans to donate to charity. But there hasn't been a kickoff event, official or unofficial, until now. Black Friday and Cyber Monday, meet #GivingTuesday.

Conceived by New York's 92nd St. Y as a way to encourage people to support charities, the event which kicks off today includes hundreds of partners among corporations, non-profits and social media, as the hashtag indicates.

Regardless of when you donate, HLN Money Expert Clark Howard has some key rules you should follow to make sure your generosity doesn't go to waste. Check them out in his guide to charitable giving.

The Christian Science Monitor also has a chart with details on the nation's 50 biggest charities, including their ratings from various vetting organizations, and how much of their expenses go toward programs.