Almost forgot about the artist; feels good re-reading his work again once in a while.
Wtf is a safe day and why does it work
It's a period of days in women's menstrual cycle where it is
impossible for the egg to get fertilized due to various biological reasons. There's a lot of misconception on how to count when this period starts and ends and I'm too lazy to go check (and compile it into English) so I won't goes into its detail that I'm not sure is correct or not. But in general, there's a certain number of days it is safe to have unprotected sex without worrying about getting pregnant. However, the caveats are:
1. It is not 100% guarantee, more like 90%? Maybe even lower than that. This is even when you count the days correctly. I mean, human body can do weird things all the time so there is always exception.
2. Menstrual cycle is never on an exact clockwork-like schedule, even for a woman who always has a very punctual cycle. It can always be off by several factors you can never know. So you'll never be 100% sure that it is 'safe' that day or not; what you have is a general idea that the day
should be safe - provided that your partner has punctual cycle. For many women, their cycle can vary from a little erratic to coming and going as it pleases. You can throw the idea of safe day out the window in those cases.
There's a certain period you can be very certain is within this safe period, and the indication is quite literally 'the period'. When she's having her period (blood coming out), that's a sure sign she's within the safe period (but then again, if that's actually period blood and not other abnormality), so it'd be pretty safe during that time, BUT, I'd say it wouldn't be a good time for neither of you for various reasons, unless you have a thing for it.
TL;DR
It's a certain period of days that is regarded as practically not possible for women to get pregnant due to biological reasons; but be warned, it is unreliable and there's a lot of misconception around it. So, don't actually rely on this method to prevent pregnancy.