BC before stims

On what day of AF did you start birth control before you started your stims? I’m due for my period tomorrow (Sunday) and my doctor just told me to call him on day one of my period and he would start me on the BC for 2 weeks before I start my meds. Since it’s the weekend, I’ll have to wait until Monday and I’ve read some protocols have you start on day 1 of AF and others just have you starting whenever just for timing purposes.