I believe it’s best to plan in days off or weeks off in advance, depending on the period you are going through at work.
For myself generally, I want to take a 2-days off at the end of every 2 months because otherwise I begin to burnout. (Thurs,Friday,Sat,Sun)
Business is a 10 year marathon made up of sprints.
However, some days you just feel wrecked. I suggest strongly that you take THAT exact day off or the one after and ensure that you re-energise.
A day off here and there can keep you powering for weeks more.
I’d rather take a day off than have to take a week or more on sick/burnout leave.
Think about the net affect.
Just do it.
If your boss thinks you shouldn’t have a day off or that you are soft, change jobs.