I go along with what the others are saying, so when it sprouted. I write the date on a piece of blue masking tape and stick it to the side of the container. Every time the plant is transplanted to another container that piece gets moved along plus I add a new piece of tape with the date of the transplant.
Most of us will not remember the actual date after a couple of weeks and if it was important enough to want to know on the day it sprouted then it is worth making the note. At least that is the way I see it.