Our birthday strictly speaking occurs when the Sun occupies the exact degree of the zodiac it was in at birth.
As the suns' position varies slightly from year to year it means it can occupy the degree it was in at birth on the day before or the day after your actual birthdate.
When this occurs your actual birthday is the day before you celebrate it or the day after.
If you wish to celebrate your birthday on the correct date all you need is an ephemeris for the current year and know what degree of your sunsign the Sun was in at birth.
Once you know the degree the Sun was in on the day you were born you can consult an ephemeris for the current year to discover when the sun is in the same degree as the day you were born and celebrate your Birthday on the correct date every year.
This is likely to be of special interest to all those born on 29 Feb as it is obviously untrue that they can only celebrate their birthday once every four years. The suns' degree on their date of birth can be checked as shown above so they will know exactly what date to celebrate their birthday each year.
I will be pleased to answer any questions you have on astrological matters and put my forty years experience to good use. It is a fascinating subject to study and very enlightening.
Kind regards John