Irregular cycle and ovulating

So my period in December was on the 28th and my period in jan was the 20 so that makes it 23 days in between. So if that's right when did I ovulate this month? If i put in a 23 day cycle it says the 26-31 is my most fertile days, is that right? So confusing...

I used MyDays app on my phone to track, and the fertile days were always right for me. I am also irregular. After trying to get pregnant for so long I noticed what my body did while I was ovulating. You can tell by your CM mostly.