Prenatal vitamins

When is the optimal time to start taking prenatal vitamins? Might seem like a silly question, but my partner and I have just recently started actually ttc. Previously we were just using no birth control and letting fate take over. Now, we actually would like to get pregnant and are starting to be more conscious about the timing of ovulation and sex. So is now the time to start taking them? Like I said, may sound silly, but I just didn't know.