From Professor John Webster:
Today is Shakespeare's (estimated) birthday! We don't actually know exactly which day he was born, but we DO know that he was christened on April 26, 1564, and some scholars have declared that three days would be a likely length of time before a birth and a christening. Thus, April 23.
But!: there is actually a quite different reason for declaring April 23 to be his birthday: we know through very clear records that he died on April 23rd, 1616--a date that would be very "poetical" (to quote Viola in Twelfth Night) were it true that he had been born on exactly the same day(!) some 52 years before!
So, given that people over time have gone sort of ga-ga over anything Shakespearean, the real reason we have embraced April 23 as his birthday is most probably owing to the fact that it would be a lovely kind of coincidence were he to have died on the very same day he was born.
This kind of time-keeping may seem, however, like something that could have come up in the absurdist comedy The Bald Soprano: It is something that is true only because people want it to be true, not because it actually is true!!!
However, it's actually even weirder than this, because in 1582, when Shakespeare was 18 years old (the same year he married Anne Hathaway), the keeper of the Official Calendar of the world, Pope Gregory in Rome, commanded that ten days be dropped from October, with the result that in 1582 October had only 21 days!
That sounds truly weird, right? So how could it possibly have happened??? (And who knew that the Pope got to be in charge of the calendar!!)
Well, this weird action was caused by the fact that the earth actually circles the sun not in exactly 365 days but in 365.242 days! This bit of astronomical weirdness is the reason we have leap year every four years--but the one day every four years that leap year gives us is not quite enough to fix the problem! So leap year (a system set up centuries before Shakespeare's day) wasn't quite correction enough, and what had happened was that over the course of many, many years that extra loss of .008 day/year had added up to being 10 full days! So Pope Gregory resolved to fix things and declared that 10 days were to be dropped from the month of October of 1582.
You can read all about this at https://www.wisegeek.com/what-happened-to-the-calendar-in-october-1582.htm, but the point here is that Shakespeare was not exactly 52 years old on April 23, 1616--the day he died. Biologically he was actually 10 days younger--(or was it older?!). Point is, astronomically speaking, he didn't really die exactly 52 years after his birth, but people have liked to think he did, so that's what has become the "reality."
The upshot??? Were Mary the (weirdly logical) Maid in the The Bald Soprano to comment on whether Shakespeare was actually Shakespeare, she would certainly have pointed out that Shakespeare could not possibly have been Shakespeare.... (?!)
Who Knows?!!! So, not really knowing for sure the date of his birth, let us, with a bit of Sir Toby Belch's Twelfth Night madness, wish the man called Shakespeare a Happy Birthday anyway!