When do I start my clear blue opk?

Hi all. I first ever post, I am going to start TTC for the first time ever with my partner of 10 years! I'm still getting the hang of what we are supposed to do and when lol.

According to this app, I am due to ovulate on 21st Sept and my average cycle is 31 days (to note my period was 2 days early last month so that threw the days out of wack!) I have 10x Clear blue OPKs to use, what day should I start? Thanks

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