Not only can you get some exercise, meet your neighbors, and be more environmentally friendly (among other benefits), you can also decrease your risk of being injured in an automobile on Sunday if you leave your car in the driveway.
As I saw on the Discovery Channel website, a study by the University of Minnesota Center for Transportation Studies shows that the 4th of July is the most dangerous day of the year to drive, in addition to the summer being to most dangerous season to be on the road. Some factors that make Independence Day especially dangerous include the amount of traffic (summer travel season and people going to cookouts I suppose) and the increased amount of drunk drivers (holidays tend to lead to a spike in drunk driving).
I'm not sure of any exact statistics for the 4th of July, but according to one source there is an automobile fatality every 12 minutes and there are in excess of 40,000 fatalities every year. Such statistics are powerful and knowing that one particular day had a disproportionate amount of traffic deaths is enough for me to say that the 4th is a good day to walk, bike, or use mass transit. Besides, the weather is great and Portland is a great place to not be in a car.