You must chart your menstrual cycles on a calendar for several months.  If your periods are the SAME LENGTH every month, you may be able to predict the day of ovulation.  Your 'unsafe days' are both BEFORE and AFTER ovulation.  Because the egg lives for 24 hours, sperm need to either be there to meet it or arrive shortly after ovulation for you to get pregnant.  You are unsafe from 7 days before your day of ovulation to 4 days after ovulation.  This is because sperm can live up to seven days in you.  They have consistently been found alive and well five days after intercourse.