Slasher films love making people in the present-day pay for the sins of the past—we already mentioned Prom Night, in which the killer’s motivation is very much in this vein. A variation on this is the re-starting of an event linked with tragedy in the past, which then becomes an emotional trigger for a disturbed soul who copes by launching a murder spree.
Two examples, both from 1981: My Bloody Valentine’s Valentine’s Day dance, which returns following a 20-year break after a deadly mining accident; and The Prowler, which sees a small town’s college graduation dance revived after a double murder 35 years prior. In both cases, well… someone ain’t happy about people having fun on such a solemn occasion, and gallons of fresh blood are shed as a result. Word to the wise: pick another day for your big dance, rather than remounting the event in such a way that pitchfork-wielding maniacs will be tempted to join the festivities.