TTC confusion

Roz

I am having a hard time figuring out when i ovulate. This ap says one thing but i did a test at that time and it was negative. Ive tested for 18 daya and finally thought i had the surge. But then tried another ovulation test brand and it was negative. My period starts on the 28th...but I havent ovulated in 18 days.

Do you start testing on day 5 of your period or what? Need advice...the internet is confusing me