September has been the biggest month for births in the United States over the past two decades. There is December nine months before September, which is most likely the month of conception for September infants. Elective births and inductions are another reason for the predominance of September birthdays. The first week of September is also when many high school athletes have their birthdays. The American Academy of Pediatrics recommends that children receive their first vaccine by 12 months of age, with a second dose of polio vaccine being given at 4 to 6 years of age. Most children receive their first vaccination by 24 months, but some countries require children to be vaccinated before they can enter school. Children's birthdays fall on September 14th or 15th depending on whether it is a leap or non-leap year. This is because this is when people start counting from zero which is how manyers calculate the date of a person's birthday. February has only 28 days so someone who was born in February has one extra day added to their birthday if it falls on a Sunday or Monday. March has 31 days so someone who was born in March has one extra day added to their birthday if it falls on a Tuesday or Wednesday. Children's birthdays were once considered important social events that allowed parents and friends to show their love and support for young people.