Twins can be born on different days, and it happens quite often. The phenomenon of one twin being born on one day and the other on the next day is generally the result of one child being born a few minutes before midnight with the sibling following minutes later — for instance, one baby is born at 11:58 p.m. and another is born at 12:01 a.m.
Rarely, a boy and girl may be identical twins. This condition, called Turner Syndrome, afflicts very few pairs of twins. The gender difference occurs when an egg and a sperm pair that begins as two males, loses a copy of its dominant Y chromosome when it splits.
An Australian woman gave birth to nine babies in 1971, but only six of the babies survived, according to CNN.com. This birth is documented in the Guinness Book of World Records.