For a few weeks each year, my sister and I are the same age until I turn a year older (10½ months older, to be exact). It's kind of funny how it went as children - people would ask my mother - how old are they? And my mother replied with the age - only one age - and people asked 'are they twins?', to which my mother replied 'No', but without explaining how we were the same age.
My mother blames a friend for her predicament as, apparently, she was told that while you are breast feeding you cannot become pregnant. Knowing my mother, this is definitely not true, because she doesn't easily believe things people tell her. My take on it is that she was quite naive in the workings of the body (despite having one child) and so had no idea of what happened during the female cycle, especially just after a birth.
My grandmother believed that she'd had a cesarian section, when actually her son was born with the aid of forceps (this is something my mother found out after she'd had 3 children of her own) and back in my grandmothers day lots of people didn't talk about women's experiences and other bits and pieces.
So my sister and I call each other 'twinnie' for a few weeks - it's quite fun really. A couple of weeks ago I'd been having tooth problems and my sister told me all about her tooth problems as well - turns out it was the tooth in exactly the same position in our respective mouths. What a coincidence.
But coincidences and shared dates are not unusual in our family. I happen to share a birthday with my husband - what are the odds? No idea. But if you factor in that my parents also shared a birthday (although not the same date as ours) it becomes more unusual. My parents married on my grandmother's birthday, one stepson shares a birthday with my ex husband, my son shares a birthday with an aunty and a nephew on the other side of the family, hubby's brother and sister-in-law have birthdays within 3 days of each other, my niece and her dad have birthdays within a few days also, my two oldest nephews have birthdays within 3 days of each other (one nephew from each sister). I guess in a larger family you could understand these coincidences of close and same dates, but ours is not a huge family.
I can only add here that having the same birthday as your husband means he never forgets. Last year I said to him 'well, this must be the least excited I've ever been about my birthday', to which he replied, 'I've got news for you.... it becomes less exciting every year!'