You could always try something like this:
I am writing to bring you some good news and some bad news. The good news is that you will be less bored in the future because you will have less leisure time. The bad news is that you will spend it at work. Commencing immediately, you will be required to work on the first and third Saturdays of every month.
Such additional work will be considered unpaid overtime, however El Jefe has generously offered to provide coffee and sandwiches at lunchtime. (Well, one sandwich, anyway. El Jefe would not want you to eat too much and become El Gordo.)
I know that you will want to discuss this with your friends. You may do this AFTER work when you are free to whine, bitch, complain, or even throw a tantrum. I don't care because I will still have my Saturdays off.
Please note that complying with this directive is completely voluntary. You are not compelled to do so. If you do not wish to give up a little of your time for the good of the company you are free to resign and look for other employment.
Anybody who wishes to appeal against this policy should do so in writing, stating your reasons. Write clearly, sign the document, then fold it until it has sharp corners and... (But I'm sure you'll work out where to put it without me being crass.)
Yours for profit,
I can see why you'd want to ask this question anonymously. >:-/