Okay, I realize I've posted similar stuff before, but it's a message that bears repetition.
It's now the latter part of July. From now until the fall cross-country season (in the U.S.) has ended, do not donate blood. I have seen athletes' promising xc seasons ruined because they were trying to be "nice" and donated, maybe during the course of a school or college blood drive.
Not only will donation directly affect your racing performance for several weeks; it will also hurt your training for a similar period, which indirectly will mean poorer racing for months.
Look, I'm a member of the Gallon Club. If you feel a little guilty about not donating, think of my having donated in your stead.
Be serious: Don't give blood, and give yourself a chance to have a great cross season!