1st April

On this day in 1975, Martin Gardner published an April's Fool hoax in Scientific American which claimed that Ramanujan had conjectured in a 1914 paper that

e^{\pi \sqrt{163}}

is an integer. In fact it is 262 537 412 640 768 743.9999999999992500 ...

The postage stamp of one of today's mathematicians at THIS LINK was issued in 2016.