At my current place of employment I have been ask at the "last minute" to reschedule one week vacations booked months in advance in each of the last three years

At my current place of employment I have been ask at the "last minute" to reschedule one week vacations booked months in advance in each of the last three years. It seems that "something" has a habit of "coming up" whenever key employees want leave. The first two times I acquiesced, but the last time was a drip to Disney with my family. I told the company it was not possible to reschedule. In retrospect, I should have told them it was not possible the first time.