I used to think that until I had a scheduler give me grief for calling her at the start of my SC to tell her. She laid into me for not telling them the day before. I bit my tongue until I could dig into the contract and be sure I was correct. I wasn't. It does say in the contract that you are supposed to tell them when you acknowledge the SC assignment. I think you are okay as long as you get in touch with them sometime the day prior.