DPO meaning..

Joelyn β€’ 29 πŸ’ Baby M πŸ₯ΊπŸ’™ / Antovelly 3/31/22 🌈

So when do you know when DPO starts? is that the day after you get a positive OPK? or? I'm trying to understand.. I had positive OPK on Monday... What dpo is that?