When should I take a pregnancy test in this circumstance?

my period in july started july 23, then in august it was a week late and began september 1. i'm not sure if my period is a week late or if it's just on the cycle of starting a week later now, so i'm not sure if I could possibly be pregnant or not. when would be the best time to take a pregnancy test since i'm not sure if I have a missed period yet. also in these circumstances, when would have been my most fertile dates this month?