How can you know which days are ovulation days?

Ovulation or the fertile period of a woman occurs around half-way through each cycle, on roughly the fourteenth day in a supposed 28 day cycle. It has been proven that in the 24 hours that follow ovulation, base body temperature rises from 4 to 6 decimal points. This is the signal on which the control method is based, marking the days on which ovulation occurs. But anyway we know sperm lives 72 hours and the egg 48hours. So if a couple has intercourse around two three times per week, there is always probability to become pregnant without using ovulation, kits, or vaginal temperature… all these mthods promote stress and very low benefit.