I ride there on my Brompton to donate and then every time get into a discussion that I shouldn’t be riding after donating, etc…
Just be sensible and listen to YOUR body. I donate when I can but follow the same approach as Gunnar and others.
On the day train first thing.
Eat and hydrate sensibly all the way up to the session. (In the UK, they make you drink a pint of water before hand too)
I then treat the return journey as a bit of active recovery and pootle back at a really low intensity.
Usually good to go next day.
It sort of depends on what you are used to. I find that works for me, others will baulk at it. But if you’re not sure, then don’t do it and save it for your off season.
Some eat cake, or drink beer at the end of their season, but a true Sufferlandrian gives away their blood?