the day before. They said to try not to for at least 24 hours so my dh had good count. They did IUI two days in a row because the day I went in for IUI I still hadn't ovulated even with trigger day before. They did tell me to bd the day after the 2nd IUI though to be safe
They told me to wait until after the IUI. So if you did an IUI in the morning by that night you should be good to go. I wouldn't do it before because as the pp said it will effect your partner's count.