Hi Amy, they will track your cycle to see when you ovulate ( depends on clinic but either OPKs, blood tests or u/s) and depending on how old your embryo is ( D2/3 or D5) will leave it that many days to put it, so it is going in to implant when it 'naturally' would. So if you ovulate cd14 it would be a few days after that probably. Hope that helps.
Just going to work so will post further if you need more info this afternoon