what day is o day

Tani • 04/05/19 💙🌈👶

so i had a positive opk at night on cd14, and in the morning and night of cd15. cd 16 in the morning the surge was gone. what cd should i consider my ovulation day? i had a 24 hour surge (tested every 12 hours) and i just don't know what day should be the day i consider the day i ovulated.