When should i test ? 😯

My AF was suppose to be here the 25th no sign of her. The the <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> changed it to the 28th still no sign I've been having light cramping here and there .. We've been TTC But I want to know when's the best time to take a test BC I don't wanna test to early and get a BFN ? Any help would be appreciated 😊