April Fool's Day: a day to commemorate Christians mocking pagans for celebrating the new year on a different, mostly-trivial point in the Earth's orbit around the sun. Also, unfunny people trying to pull pranks.
I'm not a grump. OK, I am, but really the attempts at being funny by people who aren't funny... it's just tedious. A good prank is both believable and ridiculous. Convincing people that Bathesda is working on Fallout 4, is not a prank. Convincing people that Bathesda decided to make it a point and click adventure game or a mobile game, would be a prank. The first thing is completely believable, but has no punchline. It's just a lie, there's no element of being fooled. Why wouldn't Bathesda be working on Fallout 4?
The key, really, is not believing a lie, the key is getting people to believe a really stupid lie. [Make your own religious joke here]