For a female with a regular cycle of 28 days, the safest days will be the first 14 days because that is before ovulation takes place. Sperms which enter the uterus will not have anything to fertilize because no egg has been released.
The egg can only survive for up to a day. Sperms can survive up to 3 days. So 3 days before ovulation and 1 day after will be the worst possible time to have sexual intercourse because chances of fertilization will be high.
If you have regular menses, I personally believe the best days for sexual intercourse without any form of protection would be right immediately after menstruation. However, the body works in a weird way, anything could happen. So as not to take any chances, the best is still to use contraceptions.
