I have multiple sclerosis, which is a condition where (for unknown reasons) your immune system decides to attack your nervous system. I take a medication which has immunosuppressive properties. No one at my job knows that I have MS, because even though employers aren't supposed to discriminate against people with disabilities, we all know that it still happens.
Now that the CDC is suggesting that people with certain risk factors should get another booster, I'm planning on getting it. I mentioned this to my boss because I anticipate that I will feel like crap for about 48 hours after the booster and I will probably miss some work. Still better than catching a virus and being out for a week or more. So then suddenly, for the first time ever, my boss came over to me and said “Let's look at my calendar for next week together.” So I pulled it up and we reviewed her schedule for next week. She then looked at me pointedly and said “Wow! I have a very busy week next week!”
Again, I feel like the obvious implication is that she would prefer if I did not miss any work next week. But if you go that route, when is there ever a “good week” to use a few sick days? She has no idea of my risk factors and I don't feel like gambling with my health to keep her happy. Learn from my boneheaded mistakes: don't ask permission to take time off from work and don't stupidly announce your plans, because that makes them feel like they 'get a vote' on how you use your personal and sick time.