How many DPO should I test?

Courtney • Married to the love of my life and trying to conceive our one baby to complete our small yet mighty happy family. ❤️
My fertile days are the 1st - 6th and I'm having sex at least twice everyday. 
​My period is due on the 20th, so when does everyone think I should test?
​Should I wait until the 21st if my period shows up so I don't psych myself out or should I test a couple days before hand? Opinions please :)