It's highly unlikely, but not impossible that you would get pregnant the day before your period. However, if for some reason you did have an egg fertilized at that stage of your cycle it's unlikely the pregnancy would take. The egg would not have time to implant in the uterus and trigger the hormonal reaction that stops your cycle. Therefore your body would continue with the shedding of the uterine lining and the egg would have no place to implant.