If I know the population mean but want to determine the standard deviation for a trail of several events; should I be dividing by the number of events or the number of events minus 1?

For example, I know the (true) population mean for the sum of 2 six-sided dice is 7.
A series of 6 trials generated the results #{4,11,6,9,12,7}#.
Should I use population variance and standard deviation, or sample variance and standard deviation?
I seem to remember that the reduction of the divisor for a sample was because the sample mean was only an estimate of the true population mean, but hat isn't the case here.