Hello all,

I'm working for an ESL school where the owner sent out an email lately in which she politely asked the native teaching staff if they would attend a field trip this May 1st. I understand that both May 1st and 2nd (3rd?) are national holidays over here.
Admittedly, I feel crappy about being asked to 'volunteer' my time on even one of these days because the staff and I have to work Saturdays as it is, and the holidays we are allowed to take seem few and far between, indeed, about 10 in total over one year.
The owner and admin have been known to weasel their way out of paying up teachers under varying guises from a teacher not having an appropriate receipt to warrant a paid sickday to said teacher not having signed a given paper with a particular time-date stamp to justify being given paid holiday time issued to other teachers.
In fairness to the admin and ownership, they may legally be able to justify their actions, but perhaps the results speak louder than legalities: every one of the full-time teachers at this particular campus either left before completing the contract, or of the few which did, didn't sign up for a second go.
Now on to my specific question, which is in the event that I'm told to attend one of these public gatherings on a national holiday, what action should I take that will allow me to both thwart a crafty attempt to make me work on a nat. holiday while at the same time not angering them enough to get myself in the doghouse (I'm pretty close to finishing the contract)?