Your best chance to get pregnant is if you have sex and make love regularly, two to three times a week, every week, each and every month, and once every day during your 6 fertile days (day of ovulation and the 5 preceeding days).
The exact day of oulation is often unpredictable, even when a woman has regular menstrual cycles. Because you can't exactly predict when you will ovulate and because having sex after ovulation will not get you pregnant, it's best to have sex regularly, throughout the month, especially in the week before you expect to ovulate.
Acccording to a study published in November 2000 in the British Medical Journal (BMJ 2000;321:1259-1262):
"The fertile window occurred during a broad range of days in the menstrual cycle. On every day between days 6 and 21, women had at minimum a 10% probability of being in their fertile window. Women often cannot predict a sporadic late ovulation; 4-6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle."
The authors recommend that "any couple wishing to have a baby can easily avoid the uncertainty of predicting fertile days by engaging in intercourse two or three times a week, every week."