Confused on when I ovulated.

So my period usually starts on the same day every month MAYBE a day or late but pretty much always on the same day. That would be a 30-31 day cycle length right? Glow said I would’ve ovulated on Christmas eve however from what I understand it would’ve really been on the 26? This is all so very confusing 🥲 my period started on the 10 of last month flow says I’m 12 dpo not sure what accurate. Anyone can help me understand better? I just order some ovulation test just in case it didn’t happen this month.