I fully agree that we only have ONE birthday, after all how many mothers would help their grown child reinact the event
The rest are celebrations of successfully making it another year. After all, we can't say we are "one year old" until that one year has past - we use terms like he is 6 months old, etc.
I think this gets mixed in with the planet's calendar too - we weren't into the 21st century officially until 2001 - by its very name the year 2000 says it is in the 20th century, and 2001 thus started the BEGINNING of the 21st.
But I ask though, why wouldn't 2099 be the end of the 20th century??? Seems to make more sense to call 2100 the 21st, but we all know they'd call that the 22nd!
I guess either way, it is a celebration of life and surviving another year. And just think, kids born today have a very high success rate at seeing the 100th anniversary of their own births.