Both Ruffnut and Tuffnut are worshipers of the Norse God Loki. He has a day set aside in honor of him, simply called "Loki Day". In "Shock and Awe", the Twins celebrate this holiday throughout the episode.
Based on the Twins' actions in "Shock and Awe", Loki Day is celebrated simply by playing mischievous pranks on others, which is very similar to modern April Fools' Day. It seems that there is no specific limit or level to reach the number or type of prank.