Cool calendar day

We all know (but at the crucial moment we don’t remember :) what day is today . Therefore, I propose to collect in one place (in this topic) everything that IT specialists of the whole world have prepared for us: website creators, software manufacturers with their April Fools Easter eggs, and just colleagues. Collect and get a powerful positive charge.

Let us provide links and necessary explanations in the comments, and I will transfer the most interesting practical jokes and jokes to the topic for convenience.