Can you ovulate 6 days before period due if 31 day cycle?

It's highly unlikely - ovulation usually occurs 14 days before menstruation.

Unless you use a method such as Fertility Awareness Method you cannot accurately determine when you ovulated, but in most women it is two weeks before menstruation...the only reason for suspecting ovulation closer to menstruation would be if you had a shorter than average menstrual cycle.