The most days February ever has is 29, right? While that may be true now, February 30th did exist in the Soviet Union and Sweden for a short period of time.
The Soviet Union introduced a new revolutionary calendar in 1929 with the intention of improving industrial efficiency. This new calendar featured only 5-day weeks making every month 30 days long with five or six monthless holidays scattered throughout the year. It was hoped that doing away with weekends and instead working a full month through would increase productivity. This calendar failed to catch on as it was difficult to end the day of rest tradition, and the Gregorian calendar was restored in 1940.
Sweden introduced February 30th out of necessity when trying to convert from the Julian calendar to the Gregorian calendar. The decision was made to follow the Gregorian calendar starting in 1700 by skipping leap years until the calendars were synchronized. This was forgotten in 1704 and 1708 however, leaving Sweden off track for both the Julian and Gregorian calendars. Sweden decided to restore the Julian calendar in 1712 by adding two leap days, creating February 30, 1712.