Actually, you can conceive anytime during your cycle and even during your period -- and even while using birth control (and more than one form). All it takes is one sperm to meet up with one egg.

Are you trying to conceive or to avoid conceiving?

Am actually trying to conceive.

A starting place is about 12-16 days before your next period is due. Most women will ovulate midcycle, but some will a bit earlier and some later. On occasion, you may ovulate at an off time from your own norm. So as Wondergirl said, no time is 100% for avoiding or achieving pregnancy, only that some are more or less likely to have the desired results.

There are several ways to observe when you are most likely to be close to ovulation. A more obvious one sometimes is a change in your cervical mucus. Close to ovulation it will usually become more fluid, clear, and abundant. A great book to give you all the ins and outs is Taking Charge of Your Fertility.