Fasting question

[Jack3]

Now, Good Friday is a day of fasting for those in my age group, except for those with health issues and similar series reasons.

We (as a family) plan to donate blood on Holy Saturday. I cannot fast today if I am to do that.
It's not a "must" that I have to donate tomorrow, but it's very convenient if I go with them (everything from transport to the hospital to formalities, etc)
For context, I am a frequent blood donor. Also, my family does not fast anyway.

So: is this a serious reason that takes away my fasting obligation? Is it okay if I fast on a later day instead?

[Obi-Wan Kenobi]

I would say that no, this isn't a sufficient reason not to fast. But I can't find an official statement about how serious the need must be to excuse from the fast.

[peregrinator]

I've never heard that one cannot fast the day before donating blood.