Consider the calendar. My birthday was in October, which month therefore took on special significance to me, and I learned all about it. One of the first things I learned about it was that October meant eighth month. Out there a world brim full of fools tried to tell me it was the tenth month. No amount of persuasion helped. They were willing to believe that octo meant eight, that octopus had eight legs, octagon eight sides, but October, they insisted, was not the eighth month but the tenth. I could only shake my head in dismay. Later, I learned that September, November and December had suffered similarly. Was there no end to the propensity for human error?
There were an enormous number of words that the teachers couldn’t spell either, like hors, and ayt, and ballay, and even teecher itself. In the first few books I encountered, I corrected all the spelling in them, but of course like all grownups they did not like having their mistakes pointed out to them, and I soon gave up, and tried as hard as I could to do it all in the wrong way that they wanted. Although, often I even got the deliberate mistakes wrong.
The answer is that nobody knows. More precisely, there are two different answers, along with a bunch of idiot ideas.
Answer one is that the Romans did it. Answer two blames Pope Gregory.
Some people believe that the Romans added July (after Julius Caesar and August (after Augustus) but in fact these replaced the names of existing months.
In any case, it's all about that annoying extra quarter of a day that the Earth takes to orbit the Sun. Gradually, the seasons go out of whack and an adjustment needs to be made. Either the Julians or the Gregorians (depending on which expert you prefer to believe) decided to insert January and February at the start of the year, displacing March which was the traditional first month, and throwing the rest of them out of kilter. The real mystery is why nobody has ever bothered to fix this.