My boss just emailed me stating that I was now scheduled to come in on Christmas, despite the fact that I had the day off on the schedule for a week and a half. She said it was due to coverage issues. However, she never asked me. When I tried to explain to her that I had already made plans to visit some family, she told me that the shift is now my responsibility and any call outs will result in “corrective action”. Do I have any leeway here with HR or my union? Or am I stuck working Christmas cause she says so?