Me too! (And six days after the first moon landing.)
September.
Even in my own immediate family May wins.
This thread was better when April was winning. In my immediate family, 3 of 7 were born in April.
You were born on the moon!!!11111!!!
Don’t worry. We can’t all be perfect. 
Another September representin’
For the general Spanish population, the most common month is March (that is, 9 months after the beginning of the summer), followed by September (9 months after Christmas). In Southern Navarra, the numbers pretty much don’t go down at all between March and June (the fiestas season lasts from June to mid-October).
I’m sure that same as southern Navarra behaves differently from the rest of Spain (although according to the same logic), other locations have different distributions. But I’ll be surprised if any non-equatorial location has a flat distribution.