I think you shouldn't probably do too much on the day of giving, but I've always heard its a good thing for your body.
I really couldn't find anything definitive on the net about the effects on YOUR body. But, here is what I found.
You should drink plenty of fluids in the next 24 hours, to replace the volume of body fluid you?ve just donated. You?ll also be more comfortable if you eat foods with protein or complex carbohydrates before and after giving blood and avoid fatty foods just before donating. You?ll also want to replace iron lost through the red blood cells by eating iron-rich foods like red meat, broccoli, beans and raisins.
From personal experience. It doesn't affect me that much. I give around lunchtime or in the am and try to do it on an "off" day.