When more then two girls have that special present from mother nature called the period on the same day and share the cramps and the mood swings together.
The special ritual of 2 or more close girl friends that become so close that their menstrual cycles syncronise. Once this stage is complete the girls become blood sisters by sissoring each other and exchanging their monthly discharge
Amanda: Hey Helen we've become so close over the last few months that I think we should become blood sisters.
Helen: That sounds amazing I'm due on in a few days
Amanda: Me too!