Its important to understand even if you have a cycle of 31 days does not necessarily mean that you will ovulate on the same day of your cycle.You must consider a fertile period instaed of looking for O day in case you are not charting Temp or using OPK.Looking at your cycle you should take day 12 to 20 as your fertile days.
Shannons right, Most people have the misconception ovulation occurs day 14 of your cycle its not its typically 14days before your period, If you are trying to conceive you would probably be better charting your temps and doing ovulation tests to get a much better idea, every lady is different! Good luck
Typically, ovulation occurs around day 14 of your cycle (that's based on a 28 day cycle). So yours could be around day 17.
The only way to be sure is to start checking and charting your temperature as soon as you wake up.