A lot of people in IA take friday or 1/2 of friday as a vacation day on a good weather weekend, the rest of the year can be really crappy! There is a lot of campers and boaters that like to get to those places on friday so they get a good spot, they fill up fast! I saw that all the time when I lived there. So they may be a weekend camper or boater.
Or the person did not work, worked a night shift or owned a business where they determine their own hours.
Or they work in a business that cannot work when it rains, eg: landscaping, roofing etc...it rained a bit that morning.
Or have the summer off, teacher, school guard, bus driver etc...
that is quite a big group of suspects since it occurred in the summer that it possibly could be...
People employed with the city parks dept here work 10 hour days for 4 days and take Fridays off. The fact that this happened on a Friday may be significant in the sense that this could have been done by someone that works full time but had extra time that day.