I read somewhere that, if there's 60 people in a room, you have a 90% chance of having the same birthday as someone there. Most birthdays are
clustered around just a few months.
I was actually at a convention once when I found three other people with my birthday in a span of five minutes. I was casually discussing something with a guy I had just met, and we somehow figured out we had the same birthday. When I went to tell two girls I knew about it, they both had the same birthday. It was really pretty crazy.