How do I professionally let my boss know that I will not be coming in on my day off?

It’s inventory week and we have been working long hours. Its Thursday and I haven’t had a day off since Wednesday of last week and I wouldn’t have a day off until this coming Saturday but I started throwing up this morning (just over worked and needing some rest I think as I’ve been nauseous and exhausted for days).

I called in sick and my manager texted me that I was to switch my day off with another coworker. I had specifically requested Saturday off prior to making the schedule and I reminded him that I wouldn’t be able to work Saturday and that was why I asked for it off. He said that the only day we could switch was Saturday if I can’t make it in today and that another coworker has already called in sick so another coworker had to miss an appointment because it was just the 2 of them who were able to work the registers.

Personally I don’t feel that his poor scheduling or shortage of staff should create consequences or an issue for me. I want to stick to my guns and not work Saturday. I also don’t want to go to work sick but I don’t know what to say or do.
How do I professionally let my boss know that I will not be coming in on my day off?
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