Like, seriously, people notice "OCTOber" and "DECEMber" and say, "hey, those mean 'eight' and 'ten', but they're the 10th and 12th months, what's up with that?".
If you've got a little more history, you'll know that July and August are named after Julius and Augustus Caesar, and think, "oh, they added those two months and bumped the rest of the months back."
Nope. The Romans were way, way worse at calendars than that.
July and August were actually originally Quintilis and Sextilis - the fifth month and the sixth month. They were called this because the year traditionally started in March. So they had Martius, Aprilis, Maius, Junius, Quintilis, Sextilis, September, October, November, December.
Martius was named for Mars; Junius was named for Juno. We have no idea what Aprilis and Maius were named after. (No, really.) Then they got lazy and just numbered the months.
"But wait," you ask, "what about January and February?" Hold onto your butts, because calling the months by their numbers? Not even close to the laziest the Roman calendar got.
Between the end of December and the beginning of Martius were 50-odd intercalary days. They didn't HAVE months associated with them. They were just sort of there.
I swear I am not making this up.
In addition, each month had either 30 or 31 days. I was going to say "alternated between" but I looked it up and nope, the Romans decided that was too easy, so it actually went:
Okay. This is where we are at the beginning of the Roman Republic.
Look at that. Remember it. You will look back on this and say "actually, that makes sense" after what comes next.
At the beginning of the Roman Republic, the Senate decided to fix the calendar. This was for two reasons:
1) The Romans thought the Greeks kicked ass, and wanted to emulate their calendar.
2) Count those days. You will notice that they add up to 355, which means that each year is actually ten (and change) days /shorter/ than an actual solar year - which meant that by the time of the Republic, March was somewhere in the autumn.