Same thing happened to me. I missed the morning to go to my graduation and then came back to the firm when it finished. I even missed an assessment. The firm were completely understanding – they initially told me to take off the day – and it didn’t affect the outcome at all. Just tell them early and you should be fine.
It also depends on how much graduation matters to you, but I wouldn’t miss it too lightly. It’s one of the few once-in-a-lifetime things (or two/three if you do more degrees!) that I wouldn’t miss.