The month of February usually has 28 days. Sometimes 29. How often it has 29 is the difference between the (old) Julian calendar and the (new) Gregorian calendar. The term “new” associated with the Gregorian calendar might seem strange, as Pope Gregory XIII introduced his namesake calendar as early as 1582. The adoption of the new calendar, however, was not immediate all over the world and, for instance, in Russia it arrived only after the Bolshevik revolution of 1917.
The Russian Orthodox Church still uses the Julian Calendar and this was probably the main reason why the revolutionaries wanted to dismiss it as soon as possible, as the Church was seen as both as an ally of the Tsars and as a dangerous competitor in controlling the people. You can read more about the state atheism of the Soviet Union in this post.
So, on 24 January 1918 (Julian) Vladimir Lenin signed a Sovnarkom decree implementing the Gregorian Calendar in Soviet Russia in February 1918 by dropping the Julian dates of 1–13 February 1918. Therefore in 1918 January 31st was followed by February 14th and February of 1918 only lasted 15 days, making it the shortest February in history.
As the Julian calendar “strays” from the actual length of an astronomical year by a day every 128 years, when the new calendar was put in use in 1582, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days. On 29 September 1582, Philip II of Spain decreed the change from the Julian to the Gregorian calendar. This affected much of Roman Catholic Europe, as Philip was at the time ruler over Spain and Portugal as well as much of Italy. In these territories and in the Papal States, the new calendar was implemented on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October 1582. From 1582 to 1918 the discrepancy had grown by an additional 3 days, to a total of 13. And that is why the “Boshevik adjustment” of 13 days was the greatest in history.
As the French revolutionaries had done before them with the French Republican Calendar, also the Bolsheviks wanted to create a complete reform of the calendar, rather than just adopting a widespread standard. So 10 years after the switch to the Gregorian Calendar a rather complex Soviet Calendar was created. It never really “took off” and I am mentioning it here because I find interesting how one of the basis for this further reform was, once again, anti-religious. A five days of “continuous production week” was to replace the standard 7 days week, but increased production was only part of the reason. The government wanted to eradicate the traditional names of “Resurrection” (Воскресенье) for Sunday and “Sabbath” (Суббота) for Saturday.
The Soviet Calendar only “lasted” about 10 years and it was abandoned by 1940. The Gregorian calendar has been the official Russian calendar ever since.
It is interesting (at least to me) to see how, in spite of the revolutionary fervor of the Bolsheviks, all the references to historical events dating to before the calendar reform, no matter how recent, maintained the old calendar reference. What I mean to say is that, for example, the insurrection of Petrograd took place on 25 October of the Julian Calendar, which is equivalent to the 7th of November according to the Gregorian calendar, but it always remained officially known in Soviet literature as the Great October Socialist Revolution.