December and jan are too close to Christmas, so you get all your presents at the one time and you have to wait a whole year before the next big event rolls around. Also you would either be one of the youngest in your class or one of the oldest. Feb is ok, the weather is still nice for pool parties and school has started so you have had time to make friends to invite to your birthday when you start school. March - may is great because it's been a few months since Christmas, weather is nice for outdoor parties, you can start school at 5 and turn 6, when you turn 18 it's not too close to your HSC (in NSW). June and July are cold making all birthday parties inside and I'm just not a fan of June and July LOL. Aug - october! These are my favourite months and this is where it gets crazy... I am planning on taking my children when they are 10 to Walt Disney world and aug-oct are the best months to go there and I would love them to spend a birthday at Walt Disney world ( i told you it's crazy) also the months are warming up for outdoor birthday parties in the park, it's been a while since Christmas so a burst of presents would be nice, it's after the big toy sale in June so you can stock up on gifts and save money. It also means you will be 18 to celebrate with your friends when you finish your HSC and go to schoolies and not be left out.
So there you have it my insane reasoning behind months for birthdays.