Thanks to @Techy_Margaret, I learned that donations up to $500 are being matched today at Donors Choose.
It’s the Gates Foundation that is providing the matching money. Donors Choose says the offer will be open while “funding lasts.” From what I’ve seen in the past, that usually means it will last well beyond one day.
You might also be interested in The Best Resources On – & Advice For Using – Donors Choose (Please Share Your Experiences!).