Let's see if I can be brief…
365 is semiprime (pleh), so the Egyptians (and Mayans, because aliens) divided their solar year into 360 (highly composite, yay!) plus 5 leftover bullshit days.
The early Romans (kingdom? republic? who knows) liked the idea of 5 bullshit days and copied it over, but put it into their lunar calendar because Romans suck at math and science.
The 5 bullshit days were fixed by two particular holidays, Regifugium (kind of like 7/4) and Terminalia (kind of like 12/31), and the 5 Days of Bullshit couldn't be moved or extended, so the occasional 13th month required of lunar calendars would be squeezed into the middle of February, immediately preceding the Five Days, because, again, Romans suck.
Julius Caesar finally happened upon the idea of bullying the nerds to do the Romans' math homework and got an Egyptian to fix the Roman clusterfuck of a calendar. Julius Caesar then proceeded to put his name on said homework ("Julian Calendar") and Sosigenes was consigned to his locker.
But Sosigenes Caesar knew an occasional 366th day would need to be added (something the Egyptians themselves hadn't started doing yet, and Ptolemy liked it that way), and so that extra day was squeezed into February (rather than standing alone as 1 Day of Bullshit) where previously there had been the occasional month.
And thus Regifugium (and a Christian feast that replaced it) was kept as "the sixth day before the Calends (i.e. first) of March," which is 2/24 (again: Romans suck at math) in a common year and 2/25 on a leap year.

Now go forth and be slightly more smug than you usually are!