Why do the organizers of "Take your child to work day" have this event during the school year? Why don't they have it in the summer when it won't disrupt school?

Your guess is as good as mine Shawn. When it first started, it was called "Take your Daughters to Work Day" with the idea, at least in part, being that girls would see their parents at work and see that the possibilities for their future careers. I think the founders of the program wanted the day to get noticed and get maximum press so they chose a date during the school year.

Now, it directly interferes with MCA testing which is vital for students and schools so the day is more nuisance than benefit in my opinion.

Pressure was apparently applied over the years to add sons to the day so now it is "Take your Child to Work Day". As such, the need for making a splash, at least in my opinion, is gone. The focus should be on the kids seeing possibilities for their future careers which could just as easily take place in the summer.