I'll start by noting that some people like to argue that Caesar was born on the 13th, but ... after his death and deification his birthday was celebrated on the 12th - if it's good enough for Augustus, it's good enough for me.
Caesar's year of birth is also a matter of some controversy - the always brilliant Adrian Goldsworthy favors 100 BC, I tend to prefer 102 BC for a whole host of reasons.
And whilst we're on the matter of things we don't know about the great general, I might as well mention the Rubicon. We know he crossed it. We know that the fact that he crossed it was very important, and that it was a momentous moment in the fall of the Republic ...
We just don't quite know where he crossed it. The river has changed course, and it's also quite long. I mention this as part of my 'ancient battles are darn near impossible to reconstruct because we cannot be sure of the terrain etc.' theme.
On a purely technical note, the birth was natural and did not involve a C-section - we know this as: i) it is likely to have been mentioned in the sources as a marvel, a sign of being fortune's favorite, had he been born by c-section, and ii) his mother Aurelia lived on after the birth, which was not the case with mothers who were 'given' c-sections until relatively recently. It took place in Rome, almost certainly in the family home.
JC was Gaius Marius' nephew, and helped rehabilitate Marius' reputation after a decade of libels by Sulla; in turn Marius' supporters gave Caesar a helping hand up the first rungs of the cursus honorem.