Negative OPK this am positive OPK this afternoon.

Okay ladies I need some advise. My partner and I are doing IUI with my next cycle. I've been charting and bbt+ opk.  My RE requests that once we get a + to call and schedule IUI for the next day, but if I don't get a + until the afternoon will it be too late if they don't do it until Thursday? Just a question as we aren't actually inseminating until next cycle, just want to be prepared. Thanks