Preferably cd1, but if you have only just got your thermometer I would start straight away, but don't expect anything to be accurate and just use it to get into the habit of temping each morning. I kept forgetting for the 1st week or so
I agree with Sinead hun, start now as it gets you into the habit but it wont really help you see anything until you have had at LEAST one full cycle doing it! Goodluck!