No, this is not some sort of name day of people with the surname Mancini or Mancino. It's International Left-handers Day: August 13. In Italian, "left-handed" is mancino (mahn-CHEE-noh).
It isn't entirely exaggerated to say that lefties (not in the political sense) are a minority group, as they are indeed a smallish percentage of the population and have historically been discriminated against, starting with forced use of the right hand. Even in Italian there are negative connotations: sinistra is "left" in the literal sense but also just like our "sinister" in the figurative (and bad) sense. A synonym of sinistra is manca, usually found in the idiomatic expression a destra e a manca, or "left and right" in the figurative sense e.g. he was spending money left and right. And manca is related to the verb mancare, to lack. The noun sinistro means "accident, disaster," perhaps suggesting that these folks and their awkward hands might actually be a menace to your life and limb.
But the most famous left-hander in history was not lacking in much at all. Leonardo da Vinci was known to write and draw with his left hand, although some speculate that he may have been a right-hander by birth, who was forced to use his other hand by illness (or maybe a sinistro happened to his right hand). Somehow I don't think so. Being different can also help people to break out of conventional schemes, as they are not in those schemes in the first place. He was famous (among a ton of other things) for his mirror writing, or writing from right to left, as seen in the picture.
Why did he engage in this unusual activity? Some maintain that it was simply for convenience, to avoid smudging the ink as he went along in his left-handed way. But there have been legions of lefties and very few wrote this way. Others think that it was a simple form of cryptography, to keep people from reading his thoughts. The objection to this is that it is an easy form of code that anyone can decipher. I do give some credence to this- although the code is easy to break, it does make it more difficult to read, unless the materials were stolen outright. In fact, being bilingual, I have sometimes written sensitive things in Italian while in America, and in English while in Italy, just for privacy. Any unwanted readers could translate, but it would slow them down. I imagine that in his case it was a combination of factors. But who can fathom Leonardo, even in his slightest aspects?