I was a Manager with breast cancer - and I told my staff right from diagnosis. But a lot depends on what you want - I wanted business as usual, as I continued working, but with the caveat that they had to let me know of signs of chemo brain, which happily didn’t happen. But packing up a house, moving interstate, camping out in a hotel - these are stressful without bc. And then you throw COVID in the mix. If you think you can keep your focus and preserve your privacy, then at least give yourself till the end of term. But if you need some supportive understanding or some leeway with responsibilities, then best to tell your principal but set the tone of the conversation by knowing just what you want from the discussion and whether you need that discussion to be confidential. Best wishes either way. Best