Where I work (regional airline), we must wait 72 hours after donating blood before we can accept any flight assignment.
This reminds me of a humorous story: Back when I was a new flight instructor, I was getting ready to solo one of my first students, "Alan". Alan was an older guy who had already demonstrated excellent flying skills and judgement - we were waiting only for the right combination of weather and daylight.
Anyway, it's time for Alan's next lesson and the weather is absolutely perfect. We were just about to get started when I noticed a large band-aid on Alan's arm. I asked him about it. Alan grinned, flashed his Red Cross "5 gallon" pin, and replied "I gave blood again today, I do it every three months."
Me: AAAIIIEEEE!
This is another one of those things they just don't teach you in CFI school. Why does this stuff always happen to me? I couldn't immediately find anything in my FAR/AIM about this issue, and Alan assured me that he'd never had any problems. So, off we went - and at the end of the lesson, I let him go up for a few solo takeoffs and landings. Alan was totally cool about the whole thing, and I watched him execute several flawless landings from my observation post near an Aeronca Champ.
If this is your first time giving blood, I would be very cautious about "flying all day" and/or operating at higher altitudes, at least until you figure out how how your body is going to respond. I don't think a shorter flight at lower altitudes would be much of a problem (especially if you wait 3 days).
FFFI