April Fools Day is a day when some people play practical jokes on each other. Other people are jerks and do mean things and think they're being funny, but it's not funny to pull down a website and not let people post stuff, or upload pics of their favorite politicians.
That's not funny.
Usually held on the first day of April, unless it's on the weekend.
Because nobody plays jokes on the weekend, especially if it's Sunday, which is an especially godly day, when people are serious and don't play games on each other.