Pre-Performance Apathy (and the Importance of Mentally Disengaging From Practice)

There's a famous quote attributed to Heifetz (or any number of different folks) - that if I don't practice one day, I know it; two days, the critics know it; three days, the public knows it. So does that mean that we can't afford to take any days off, if we're truly serious about our craft? And what does it mean if we start to feel burned out and stale? Well...it could be my imagination, but I swear I once saw a video in which Heifetz said that he always took one day off from practicing a wee...