Missed period but BFN

My husband and I have been TTC for 8 months now and I have always had me period on time like clockwork. I was supposed to start my period Monday according to this app but if I went by past periods it would have started last Friday. I still haven't started and there are no signs of AF starting but I have taken to tests and they are both negative. When should I try again
So that I don't keep getting my hopes up?