Day 1 starts on arrival. You have to leave (or at least, pass through passport control in Minsk Airport) by 23:59 on Day 5.
Belarus had 5-days visa-free in Polish border area before, has anyone used it and have an idea?
Yes. There are two zones, one around Belovezhskaya Pushcha and one around Grodno and the Augustów Canal. They have slightly different requirements, but both require advance booking online to obtain the visa waiver. It's painless, but you have to be careful as, for instance, the visa free regime does not
apply to the Augustów Canal zone if you travel by train.