The habit to smoke weed only on 3 days a week and with this act defining your personal 3-day-weekend at the same time.
On the other hand it's a rule, which helps stoners to keep 4 productive days to live the creative ideas the weed consumption in the other 3 days gave them.
The rule descents from the seek of balance, the constant lookout for keeping Ying and Yang in whatever sort of equilibrium.
If the rule is applied in a strict way, a day counts as as weed day whenever a first consumption takes place no matter what time of the day it is.
I tried straight edge some time, but now I am doing the 3-day-weed-week.