You’ve probably been saying happy birthday a lot lately

September is the month with the most birthdays.

Top quiz: what do Hugh Grant, Michelle Williams and Tolstoy have in common? And what do Colonel Saunders, Otis Redding and my mum have in common? Birthdays! All were born on September 9.

“It’s crazy,” said my mum when I called her on Saturday to sing the birthday song.

“Every day this week it’s been somebody’s birthday.”

She’s right. If you haven’t said happy birthday to someone lately then you’re likely a hermit or a Jehovah’s Witness, because September is officially cake month.

According to US stats, nine of the 10 most popular birth dates fall between September 9 and 20. And September 9 is the most common birthday in all America.

Yes, if your birthday is round about now, your parents were getting jiggy at Christmastime. You are officially a mistletoe baby.

However, Christmas Day itself proves the least popular birth date, presumably because doctors are on holiday. But these stats are American, which got me wondering about South Africa, and finally lead me to a website – limn.co.za – run by a Johannesburg engineer called Jeff Fletcher.

For a project, Jeff had access to the voters’ roll, and therefore a large list of identity numbers, and he realised with these he could work out the most common birth dates in South Africa. The results proved startling.

September did have a lot of birthdays, but the most popular single birth date was January 1. Yes, New Year’s Day, the first of the first, had over twice the number of birthdays as the next most common date. And then, oddly, the second of the second month, February, had a lot. And March 3. And April 4. And so on until December 12.

Luckily, Jeff has a hypothesis for this, and it breaks your heart.

Prior to 1994, a large proportion of South Africans had no birth certificate or ID book.

In the rush to register to vote, those who knew only their year of birth seemingly gave, or were given, January 1 as a birthdate; those who knew the month were given the corresponding day of that month, for instance the fifth of the fifth.

And then, rather beautifully, there are a huge number officially “born” on 16 June, Soweto Day, forgotten by apartheid, but born again as proud South Africans.