Help!

Hi all. My last period was around the 2nd-5th of August this month. I’ve logged in this app that it was the 1st-5th as a guide as I really can’t remember. It was my first period of the pill and it’s pretty irregular as normally they’re 6-7 days and quite heavy. We TTC at the end of this last week as it said Thursday was my highest chance so fingers crossed. My question is how will I really know when to take my first test as my period has been irregular?? I won’t really know roughly when to take it? Thanks!