The Big Picture 
Many of us, in parenting, have a tendency to "sweat the small stuff" too much. While it is indeed possible that a child skipping school for several days might miss a crucial unit on long division, typically kids do catch up when they miss school due to strep throat or chicken pox.  

Interestingly, according to the TIA travel poll:

These figures seems to paint a picture of well-educated parents who believe that the gains from family travel experiences may justify some truancy. 

Consider the teachers too
Finally: if you do decide to travel during school days, remember to consider your children's teachers, and the extra work that your child's re-entry to school may mean for them.  

And definitely, for any extended absence, discuss the trip with teachers before you go: perhaps your kids can make a presentation, or do a project, based on their travels once they're back in class; perhaps the teacher will want them to do certain work-- in math, for example-- while they're gone.

By and large, my kids' teachers have been open-minded about school-time travels, but we did have one situation where my Grade Five son paid some heavy dues after a five-week trip in Europe. (A teacher had sent along a formatted journal which caused so much stress it was spoiling our trip. I finally said, "Stop the journal, I'll explain to your teacher"-- a strategy that didn't work. ) 

Bottom line: talk to your teachers; weigh your pluses and minuses; and then step back to see the Big Picture. Am I glad we took that five-week trip to Europe? You bet. And our kids are still  reaping educational benefits, in school projects etc.