Posts that commemorate [[April]] 1st, [b]April Fools' Day[/b], usually celebrated by presenting very elaborate pranks to someone else. 
These posts either contain the words "April Fools" (エイプリルフール) or a picture that the artist intended to make as a prank, especially if it is obvious that what is depicted is very absurd.
エイプリルフール 愚人节