Dean plans the day with all the precision and care usually dedicated to a large-scale military assault, scoping out not just the predicted weather conditions (muggy, overcast) and the parade route (four blocks over from the place he and Sam are squatting in; there's a stretch of five houses that'll be empty the day of, so they can set up there for decent seats), but also local church potlucks, which towns have the best firework displays and when, and whether or not Dean can get his hands on some pyrotechnics of his own (a milk-crate full, in lieu of a week's under-the-table payment at the local body shop; good thing peanut butter's cheap).
His father would probably have words about how he'd appreciate it if Dean would apply the same dedication to researching a hunt, but he's with Caleb two states over, trying to track down a wendigo in the middle of backwoods nowhere, and this is Dean's one chance to give Sammy a perfect Fourth.
The parade's not quite an hour long, but there's people throwing candy from the back of some very nice old convertibles, and the high school's cheerleading squad does flips in front of where he and Sam are eating very melty fudgesicles Dean nicked from the freezer in one of the empty houses, so it gets a decent score from him. Sam's transfixed, nearly incandescent with happiness. Someone handed him a stars-and-stripes pinwheel while they were walking over, and he doesn't put it down until they're sharing someone's blanket at the church potluck--not even he can manage a sloppy joe one-handed.
That night they'll sort of set a field on fire, but everything is damp still from the previous day's thunderstorm so it won't spread, and Sam's smile will shine so bright Dean would be willing to burn down an entire state just to get him to smile like that again.