Forced to work holidays when not my scheduled days

My wife works in the guard shack at a private lake community. She works first shift weekends, every weekend(she works Sat & Sun 16 hrs every week), on time never misses, she has always worked any holidays that fall on the weekends for time and a half. After the fourth of July they told her they would not pay weekend employees time and a half any more for working holidays.
At times she fills in for other shifts during the week if they have vacation, etc. She used to work some holidays during the week for other people during the week when asked and would be paid time and a half. For cost cutting measures the lake manager is now making the full time employees who work during the week take off on holidays and is requireing them to schedule the weekend workers to work the during the week holidays without time and a half pay. They are threatening to fire any weekend employees who will not work these during the week holidays.
I presume they do not have to pay employees time and a half for working holidays? My question is can they fire a employee for not working a holiday that falls on a day that is not an regularly scheduled work day (ie. during the week)?