Cool day of the calendar

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

Let's give links and necessary explanations in the comments, and I will transfer the most interesting jokes and jokes to the topic for convenience.