Day before but your points get into holding since it was canceled less than 30 days prior. I think holding is still waived, but no one knows for how long.
I was on the phone with MS this morning (a surprising short 30 min wait) to book a reservation and ask my question. They said I have until day before to cancel under their temporary cancellation policy. If you use borrowed points, they would be return to original use year within 15 days. Of course, they won't know when it will end but most likely to be extended into 2021.