The calendar date February 30th has happened just once in history: in Sweden in 1712.
Every four years, February has an extra “leap” day. And every hundred years, February does not have a leap day. But every four hundred years, February does have a leap day. In other words, this wacky month oscillates between 28 and 29 days. But once, just once, February had 30 days.
Pope Gregory XIII introduced our modern leap day rules in 1582 to account for the discrepancy between the calendar year (365 days) and the solar year (365.2422 days). His Gregorian calendar corrected a flaw in the earlier Julian calendar. The Julian calendar included leap days every four years without exception; this led to a gradual calendar drift over centuries. Astronomical events like the Spring equinox no longer lined up with their expected calendar dates, and this problem increased over time.
Correcting time from the Julian to the Gregorian calendar was fairly simple: on the year of conversion, you just skipped ahead ten days. So, in France for example, there was no October 10th, 1582 – that day, along with nine others, was simply jumped over entirely in the conversion from the old Julian to the new Gregorian system. But this was complicated by the fact that not everyone converted to the new calendar at the same time, or in the same way.
The Protestant countries, for example, mistrusted the Catholic Church enough to avoid their Gregorian calendar for a long time – in some cases, full centuries. But eventually everyone in Europe moved to the new system. When they did so, they jumped ahead ten days (or later, eleven days to account for the additional drift from their delay). Well… most of them did, anyway.
Sweden got around to calendar reform at the turn of the 18th century CE. But it had its own plan. Rather than jump ahead ten days at once, they would simply skip every leap day for forty years. After forty years, they’d line up with all the other Gregorian countries – but in the meantime, their calendar would be in sync with no-one else.
In 1700, there was no leap day in Sweden. Then, war. The Great Northern War – between Sweden (and others) and Russia, Denmark-Norway, and Saxony – distracted everyone enough to ignore the plan. Sweden had normal leap days in 1704 and 1708. In 1711, the King of Sweden abandoned the plan entirely: Sweden would return to the old Julian calendar. There was just one problem. They had already skipped one leap day, so a return would require it to be returned.
And now, at last, we get to 1712. Sweden, in an attempt to return to the old Julian calendar, had two back-to-back leap days. For the first, last, and only time in history, February 29th was followed by February 30th. It was a Friday.
Sweden did eventually switch: in 1753 they jumped forward eleven days, which finally put them in sync with the Gregorian calendar.