IUI and follicle rupture. Immediately?

Hello doctors we are ttc for 6 months and our gynec yesterday has planned for asked us for IUI tomorow if there is a rupture and asked to get follicle scan today . We got the scan done just now and it showed 21 mm on left ovary and when contacted she said it will take few more days to get the egg to rupture and scheduled for Monday . She asked to get HCG injection tomorrow between 3-5 pm. Sunday is a holiday so I'm just worried that the egg rupture can happen tomorrow or Sunday and we might be late for IUI if we do it on Monday . Will HCG affect rupture immediately? Is 21 mm good enough size and it can rupture .? It is 12th day of period today . I'm just concerned that we might miss the timing of iui. Kindly advice based on situation. Thank you.