i gave blood for the first time in a dozen years about a month and a half ago. make sure you're hydrated, take it easy immediately afterwards, but you should be able to lift again by Monday. and don't give blood on a full stomach.
i actually felt faint about 9/10 of the way through, probably b/c i wasn't sufficiently hydrated & ate only an hour before donating. if things feel wrong, tell 'em, and they'll get your feet above your head. i felt fine again, then stood up, grabbed a sprite & crackers, and sat down -- and a few minutes later felt really, really faint. never felt like that before, not even squatting (companion said my lips turned white). they laid me back down and i was fine in a few minutes, and left w/o further complications after about 10 minutes sitting up straight.
soooo consider skipping your friday workout, or putting the donation way after, certainly after you've digested any PWO nutrition. you can ask them how long to wait before lifting again but they'll give you an unnecessarily conservative estimate, IMO.
the donating is good for you though, to get some iron out of your system. the ladies do so once a month
there's less pain than getting a shot (I got three shots recently since donating blood, so the comparison is fresh in my memory) and there's zero risk of contracting HIV.