September, October and November are officially New Zealand's 3 biggest birthday months. Here's a few fun facts: 30 September is the most common birthday the 10 most common birthdays all appear in the 10-day period from 22 September to 1 October the least common birthday is 25 December (apart from 29 February, which only occurs in leap years) the least common birthdays tend to be around public holidays: 24–27 December, 1–2 January, 6 February (Waitangi Day), 25 April (ANZAC Da