why does february have 28 or 29 days when the other months have 30/31?

Leap year because it jump over one day every four years. That's because in 365 days a year there is a little bit more hours every years and when you add these hours it makes one day more every four years.

As a joke, that's why a person born on the 29 of February, has his birthday only once every four years!

Febuary has 28 days in a month except for leap year which has 29.