Creamsicles are popsicles with ice cream in the center, and I think most people think of the vanilla ice cream and orange flavored popsicle combo when they hear "creamsicle." Creamsicles appear to be very popular and can be found across the country. As with any popular food item, there are many recipes for homemade versions (some with grapefruit flavor.) This recipe made for a Zoku (but can really be used with any popsicle mold) uses buttermilk to get the creamy flavor. I don't think any ice cream molds exist yet that will allow for homemade versions to be like the real thing.
Also as with any popular food item, there are plenty of grown-up drink recipes that try to capture the flavor. This recipe makes a whole mug of a very close match using whipped cream vodka. If a whole mug of drink isn't your style, try making jello shots instead. For a super-easy jello shot recipe, check out this 3-ingredient recipe.
According to WiseGeek, today is not an official "National" day because there has been no official government action to deem it so, but August 14th has become known as "National Creamsicle Day." I think we can thank the good folks over in the Unilever publicity department for that.