"Hi, this is the Stanford Blood Center, and we're calling to see if you are available to come in and donate in the next few days?"
(the way this is phrased is a little odd, not like their usual reminder calls)
"Actually, my husband and I are already planning to come in tomorrow, since we've both been eligible again for a few weeks."
"Oh, thank you! I'm sure you think your blood type is both a blessing and a curse."
(I'm O negative, the universal donor.)
"I'm used to reminder calls, it's fine."
"It's just right now we have less than a day's supply of your type on hand. Thank you again for coming in, see you tomorrow."
Spring break is doing a number on blood banks all over, too many people traveling or simply too busy to donate. If you are willing and able to donate (and there are a lot of very good reasons why folks can't or won't) consider going in sometime soon. They'll be very happy to see you.