Definitely probably better to wait out the 24 hours to let your body recover. Everyone’s body reacts a little different after giving blood. if you have to hit the gym same day Obviously better to get your meals in and be hydrated, Generally speaking you may feel fatigued, low energy, nauseas, etc, I have donated 6 - 8 times. Once donated about 9am, felt great. lifted same day about 5pm, tons of food and water in me leading into the afternoon, didn’t have any problems.