Yes, other industries will receive economic boosts through recognized holidays. I believe that an ice cream purchase will lead to other, usually ice cream related, purchases. For example, a customer purchasing ice cream at a restaurant or other food service establishment may choose to purchase other food items during their visit. Another example is that of a customer buying a carton of ice cream at a grocery store or other food retailer; while shopping for ice cream, they may make other purchases at that store.
In this generation, if it's not on facebook - we don't know about it. So, just because an industry designates it National Chocolate Day, unless the various forms of social media actively promote it there won't be enough attention to stir up an considerable amount of business. Even so, that doesn't mean people won't make their own chocolate at home instead of putting money into the economy.
I don't think that other industries would receive economic boosts through recognized holidays, nor should they. Having recognized holidays, like for example a national popcorn month devalues existing holidays like black history month, can we really put popcorn, ice cream, or any other industry on par with the sacrifices of the likes of Martin Luther King and our fore fathers, I think not.
People do not need a national ice cream month in order to enjoy this dessert, especially in the warm months of summer and spring. It is not clear what naming a month for anything, especially a food, actually does for that item, so it certainly will not mean that other industries will need this.