I rode with one of my regular ride buddies on Sunday. He had donated blood on Friday. He is usually quite a bit stronger than I am, but I could tell he was lagging. It could have been a physical result, or it could have been psychological. But his performance was definitely not what it normally is.

You say you are a regular blood donor. How do you usually feel after donating? You know your body best. But unless there is a specific reason you need to or want to donate on that day, I'd recommend that you reschedule until after your metric.