Personally, I think 8 days is a bit much to be out of school, especially at that time of year. I'm struggling with having my ds miss 3 1/2 days in October (especially since that 1/2 will still count as a full day missed).
I used to teach first grade, and that is such an important time of year. First grade is a huge adjustment, especially if kindergarten isn't full day where you are (they are phasing it in here). It took a good month or so to get the kids used to the full day program, the new rules and expectations, new friends, etc. Also, we used to assess the kids during the first couple of weeks, and then dive right into reading and math groups that 3rd or 4th week. I just think that 8 days of missing all of that could put your ds at a disadantage. Also, it might be difficult for the teacher to prepare work for that long, especially since he/she doesn't know the rate at which the children will move (since the "real" work has just begun).
I would certainly recommend following the advice of pp and give your child a head start over the summer.
I am so tempted to move our vacation to Sept. to take advantage of that
free dining!!! Why couldn't they extend that by 2 weeks?? I think we'll stick with October, though. I just don't think missing the 3 1/2 days at that time will have such a huge impact. We'll still work hard to make up the missed work (and having ds in 3rd grade..I know we'll have plenty to do), but it's something we're willing to do as long as the kids continue to do well in school.
Have fun, whenever you decide to go!!!