I'm giving blood at the end of next week after only giving once in my early 20s. I was wondering about cycling earlier in the day and or the next day.
Cycle happily in the morning. Just make sure you are well hydrated - as I mentioned above, I have to drink a couple of pints to give a decent donation - so I guess I am mostly donating water.
I wouldn't
commit to cycling the next day. See how you feel, and don't push yourself if you decide to do it, and if it's hot be very careful. Fainting is a possibility, and that could be catastrophic on a bike.
I've found I bounce back well, and happily make appointments that require a 40km round trip. I do though, overstay my welcome in the refreshment area and abuse the biscuits, as described. I didn't do it when I first came back, after a long absence - though I was a regular donor, giving monthly plasma donations for a few years, apparently Melbourne's primary source of tetanus antibodies for a while <shudder>
amusing (well, to me) fact: I can't spell donor. Before I fixed, I was calling my self a kebab (doner)
Edit: I also can't spell tetanus