When to test

So I no this app isn't always spot on . But usually when it says I'm suppose to start my period i do . Well it said I was should have started Monday and now where Tuesday night and nothing so really it's only a day and a half behind . But I'm a little worried considering I'm in a wedding and scared I'm going to need to get my bridesmaid dresses altered to fit over a belly if I happened to be .. the wedding Is the bringing of June would I be okay ? And if I don't see me period tomorrow morning is that an okay time to test .. I'm just getting nervous I already went for my alterations. Thanks in advice for any advice or help