Check the parish calendar! When scheduling, check and re-check the parish calendar. And remember to write what you are scheduling on the parish calendar. I have in the past not checked the calendar carefully enough and had to reschedule plus shake up those I scheduled on top of. And I still have to be super sure I write my own bookings on that parish calendar. I have forgotten to do that, too. Only thing you can do when it happens is to apologize to all concerned. But also realize it is not the end the world even if you get reprimanded for it. Keep your spirits up, see it as a lesson learned and know that tomorrow is another day. Also, it might increase your compassion when someone else does it to you.