One of my DCs schools has recently changed the date of an inset day that was originally secuduled for the beginning of January.
I would write a letter to the school advising the school that the child would not be present on that date, due to the fact that certain plans had been made based on the information that the school had previously provided.
I would also include a copy of where I had received the information (about the inset day).
I would not accept an unauthorised absence and I would inform the school that I would make a written complaint to the governors if that was the route they were going to take. (This reaction would depend on the response that was received from the school.)