When you ovulate, it will always be 12-16 dayd PRIOR to your period.
There may be months when you don't ovulate. In that case, your uterine lining may shed (not technically a period but will seem like it) early or late causing a cycle length difference.
My last anovulatry month (didn't ovulate) was a 72 day cycle.
at 3:27 PM on Mar. 25, 2011