Longer explanation:

Because most computers use two's complement to make negative numbers. To produce -x, you take x, flip all the bits, and then add 1. Conveniently, this process works both ways, so if you have an int with a positive MSB, i.e. 1*******, that's a negative number, and if you invert and add 1, you get the positive number.

So if you take 11111111, and apply this process, you get 00000001, which equals 1. Thus, 11111111 = -1

Secondly, the gestation period of humans is 9 months, and there are 12 months in a year.

So if you have binary candles and all of them lit, that can suggest, which it does in my previous comment, that you're celebrating a child's -1st birthday.