Usually February is a very short month, with only 28 days. But every four years, in what's known as a leap year, it has an extra one - giving it 29.
Have a look at our guide to leap years and let us know your thoughts at the end.
According to the calendar, there are 365 days in a year.
But it actually takes the Earth almost 365 days and six hours to revolve all the way around the Sun.
So every four years, about an extra 24 hours have added up, so one extra day is added to the year to keep the calendar in tune with the Sun.
It takes slightly less that 365 days and 6 hours for the Earth to go around the Sun, so to make up for that, a few years aren't made into leap years even though you'd expect it.
A person who was born on 29 February is sometimes called a leapling.
And although their birthday only actually happens once every four years, they don't go without presents!
In normal years, leaplings celebrate their birthday on 28 February or 1 March instead.
A year that can by divided evenly by four is usually a leap year.
February 29 is the 60th day in a leap year of the Gregorian calendar, which is the most widely used calendar in the world today and the one used in the UK.
Traditionally, it was seen as the man's job to ask a woman to marry him.
Women could propose themselves only on a leap day.
We asked what you're doing to celebrate and how old you'll actually be today.
"My friend has been alive for 8 years but is only 2. This is a very special birthday for her as it's only her 2nd. She normally gets more presents than anyone else because she's special!"
Jenna, 11, London, England
"My dad is a 44 year old leapling! He is celebrating his 11th real birthday and next time he has a proper birthday I will be older than him! He doesn't mind because on the other years he has two birthdays like the Queen."
Elizabeth, 9, West Yorkshire, England
"My cousin has just turned 12, she usually has to have her birthday on March the 1st, but she really doesn't mind because it's Justin Bieber's birthday then too!"
Saimah, 14, London, England
"My maths teacher was born on a leap year, so she's only 7 years old! Our whole class is older than her!"
Ella, 12, Portsmouth, England
"My friend's birthday is on the 29th, and so is her mum's. Her mum is 36, but she's really 9 and she even gets cards which say 'Happy 9th Birthday'."
Kirsty, 11, Cambridgeshire, England
"I am 13 today, although I am really only 4. I like having my birthday on a leap year!"
Rose, 13, London, England
"My Dad is 38 and he has to have his birthday on the 1st of March, which is weird. He gets presents on both days though."
Omar, 13, Manchester, England
"My friend's birthday is on the 29th. He's 12 but only 3 in leap years! He is going to have an extra special party with it being a different birthday to normal."
James, 11, Northamptonshire, England
"It is my birthday today. I am actually 2 but aged twelve. I'm going to celebrate my birthday with my friends and family, it will be so fun!"
Katie, 12, Glasgow, Scotland
"Its my cousin's birthday, he is 21 but is only 7 in leap years - younger than me! We normally have his party on the 1st of March."
Alina, 10, Oxford, England
"Today is my birthday and I'm 12, but really 3 in leap years."
Ayesha, 12, Northamptonshire, England
"My friend is only 4 today (even though he is 16) because he was born on a leap day! It's really good because he gets a big party every 29th of February!"
Callum, 12, Kent, England
"My friend is normally 13 years old, but her birthday is on the 29th of February, therefore she is only 3 years old."
Allessandra, 13, Amsterdam, Netherlands
"I think that people who have their birthday on the 29th of February are a little bit unlucky because they only have a birthday every four years."
Anna, 10, Hong Kong, China
"It's my great uncle's birthday today, he is now 13! That makes us almost the same age!"
Harriet, 12, Buckinghamshire, England
"It's my best friend's grandad's birthday today and he is actually 16."
Willow, 10, Isle of Man, England
"It's my 12th birthday today even though I'm really only 3. I like having a leap year birthday as it's extra special."
Jack, 12, London, England
"My Mum is 40 today! To celebrate we have put out balloons and banners even 2 chocolate cakes!"
Tash, 10, Crewe, England
"It's my special birthday today. Although it's eight years since I was born, I am actually only 2 years old today. I normally have a pretend birthday on the 28th February."
Lacey, 8, Barnsley, England
My friend from school has her birthday on the 29th of February. She loves it because she normally has her birthday on the 1st of March but when the leap year comes she get presents on the 29th and the 1st."
Anna, 10, London, England