Least Fertial?

Don't u mean Fertile?

If that's what you mean.... it depends on the girls cycles.
Depending on when they ovulate. IT used to be thought that everyone ovulates on cd14. So they are fertile before during and 24 hours after they ovulate.
Every month can change too... stress, and illness can delay ovulation or make it early.. and sometimes its just late or early.

Sperm can live up to 5 days (even tho 5 days isnt very typical..)
So you can have sex.. 4 days before the day you ovulate and still get pregnant. The egg, when it comes out of the ovary... dies 24 hours later. SO within that 24 hours the sperm has to get to it.

HOpe that helps.

This is a link to a web site that has alot of info on your fertile days etc
http://www.woomb.org/bom/rules/index.html