Since summer just ended you could send out a memo now saying that come next summer the following changes will be in effect as of June 2013 that there is no additional time off - book vacation when I do or pay accordingly and that effective Sept 2013 the daily rate will be. Then they have a year to plan, question, etc. Adding a reminder come March when you set your holidays for the summer should be enough. Many will need to book their weeks that early so you will need to decide when you will be closed.