Which year was the only known actual use of the date 30 February?

They omitted February 29 twice but used February 30 only once. So they still were off one day. This happened in Sweden, what about the rest of the world? I am confused.

Feb. 29 used to be Sadie Hawkins I think? In the U.S.

I myself did not know why they called leap year leap year that was very educational