When does Week One of Flowering Begin?

when you switch to 12/12 you begin your flowering period.

Whenever you want to start it.

flowering starts after the flowers become visible(male or fem).
this can be pre flowers in veg mode or 1 week after switching to 12/12.
if there is no signs of sex in late veg or early 12/12 wait untill you can see signs of sex before you assume its flowering regardless of light times.

best way to know that is use day 1 of 12/12 as a guide for how many weeks, but get a scope to check out the trichomes to let you know when they are truly ready. if you want couchlock, get those trichs 50-70% amber, if you want a heady high, cut when trichs are cloudy and maybe 10% amber