I donate regularly, biking season or no. But I don't race, so my riding isn't highly pressured.

I try not to ride the day I donate (before would be fine, but not after), but do an easy ride the next day. And after that, I feel pretty much up to snuff. Your iron has to be pretty high to donate, so your body is able to use iron appropriately. (That is, you're not anemic and so if you take some extra iron, that should be good.) You probably won't have replaced all the blood cells, but your volume should return to normal quickly, and you'll be well on your way to being full up within a week or so.

I doubt you need to be told, but here goes: drink lots of water (or your other beverage of choice) and eat well. (I like to think there are no calories in post-donation cookies!)

And finally: Thanks for donating! You know it's important and helps people lots. Thank you!