If a woman's egg is only around for 12-24 hours for fertilization and if its the most fertile time in the cycle, what is meant by fertile days?

Ovulation day: Even though most women have regular menstrual cycles, ovulation can vary from 1 to 2 days. Fertile days mean the time when potential conception can occur. You're exactly right concerning the fertilizability of the egg, but at times it's very difficult to pinpoint the exact moment of ovulation.

Some Guess Work: The least likely time to conceive is from day one to day seven. If your cycle is different from 28 days on average you have to adjust the calculations – the important thing to remember is that the egg is produced about 14 days before the next period starts. Sperms can live happily 48 hours in the vagina or the uterus so the most fertile time is day 12 to day 16.