The ‘safe days’ method is suitable for people who don’t like any other form of contraception, have sex only occasionally, or who do not terribly mind getting pregnant. Of course, they must also be able to do simple sums.
If you know what the most risky time of the month is, you can avoid pregnancy without using any contraceptive. This method -also known as ‘rhythm’ or ‘biological’ method- became unpopular with sex educators since the invention of the pill, but it is certainly not as useless as it is sometimes made out to be. It is also good to be informed about the monthly cycle. So first read about menstruation and ovulation.
The unsafe days are the five days before ovulation (because that is how long some sperm cells can survive in the body) and the day of ovulation itself (because the egg cell is fertile only then). So six days a month are unsafe, the other 24 are not. The only problem is that ovulation cannot be precise predicted and may vary for different reasons (stress or illness, also young girls and older women are often very irregular).
So it is first necessary to know how many days -on average- your menstrual cycle is and how regular it is. If you note down the first day of the period for twelve months, you have a good basis for using the table on this page.
You now understand why this method is not very good if you have irregular periods. And also why it is possible to conceive in the first week, while one is still menstruating. But of course that is quite rare.
In fertile women, an egg is released once a month, this is called ovulation. The ovum (egg cell) must be fertilized within a day, otherwise it will die, and after about 14 days the inner layer of the uterus will break down, causing bleeding, a.k.a. menstruation. To determine the moment of ovulation, you should abstain from having sex for five days before ovulation (because sperm cells can live in the body about five days) and one day after ovulation (because the egg cell lives for 1 day). So all other days of the month are safe.
There are fertility devices, the best-known is called Clearblue.