And you thought young adults were the most secular segment of the American population. According to a BIGresearch survey for the National Retail Federation, 18-24-year-olds are the people most likely to celebrate St. Patrick's Day. Seventy-two percent said they plan to do so, spending an average of $42.20 apiece in the process. Overall, the holiday is forecast to generate $3.64 billion in consumer spending. That's down a bit from last year's $3.76 billion. The report blames this downturn on the fact that St. Patrick's this year falls during Holy Week (i.e., the week preceding Easter). In all, 46 percent of adults said they'll mark the occasion in some way, including the 31 percent who plan to celebrate at a bar or restaurant and the 18 percent who expect to attend a private party.