How do I know when i ovulated?

So my fertile window was from Sep. 27- Oct. 3. I didn't take any OPKs (which i should have)and I'm not sure when I ovulated . Glow gave me a 33% chance on Oct. 1st ( I'm not sure if because i ovulate that day or because i ovulate the next day but it gives me a better percentage to have sex the day before ovulation) by my count with splitting a normal 28 day cycle counting 14 days since the first day of my period i got Oct. 2nd. And another app told me the 3rd. I'm so lost!?