The moment a grower switches the light schedule from 18 hours on / 6 off to 12 hours on / 12 off to induce flowering in photoperiod cannabis.
Jeremy is doing it today — he sets the Niwa controller to 12/12 for the whole 10x10 tent at once. He counts day 1 of flower as the day the clock actually runs 12/12, not the kickoff day.