In Buffy the Vampire Slayer season 2, a holiday known as the Night of Saint Vigeous is introduced — as it seems important, the fact that it's never again mentioned or celebrated on the show creates an interesting plot hole.
Season 2, episode 3, "School Hard" was an important episode for Buffy the Vampire Slayer, as it served as the introduction of two of the show's most iconic and beloved villains: Spike (James Marsters) and Drusilla (Juliet Landau). In "School Hard", Spike and a group of vampires plot out an attack on Sunnydale High School during parent-teacher conference night. The episode is not only popular with fans for its introduction of two major characters, but also beloved for featuring the moment when Joyce Summers (Kristine Sutherland) stood up to Spike to defend her daughter—the Slayer—against vampires while wielding a fire axe. Though Buffy and Joyce did have an occasionally tumultuous relationship due to her being the Slayer and her mother not always understanding, there was always a strong undercurrent of love between the two, which made Joyce's death on the show even more tragic and difficult for fans.
Spike's reason for being in Sunnydale is partly for the celebration of a holy and ancient vampire holiday, The Night of Saint Vigeous. As its brief mention on the show explains, the holiday is observed by vampires, who prepare for it by fasting and prayer in order to build their hunger and bloodlust up to such a extent that they are essentially feral when they go out hunting for the feast night — the holiday is also sometimes called The Feast of Saint Vigeous. Vampires are supposedly more powerful during the holiday as well, and the initial celebration was intended to hunt the Slayer. Spike led the charge a day early, and his efforts were carried out in part by surviving members of the Order of Aurelius, the group led by The Master, whom Buffy killed in season 1. Initially, it was said that whoever ended up killing the Slayer would take The Master's place in their Order, but—hasty as always—Spike acted rashly and had other plans. Even so, as the holiday fell on a yearly basis, it's interesting that it was never mentioned again.
The Night of Saint Vigeous likely just became a plot point that wasn't worth revisiting for the show. Given Buffy the Vampire Slayer's monster of the week format, the set-up for "School Hard" was just another way for a group of vampires to hunt and temporarily endanger the Slayer and her friends. While it added an interesting bit of lore not only to the Order of Aurelius and vampires in general, giving them an ironically religious backbone, it wouldn't make sense for one episode in every season to deal with the same plot that would, inevitably, be easily overcome.
The original plan for Spike's character was initially different—and worse—so it makes sense to have what was intended to be a one-off holiday be as disposable as he originally was to the show. Spike's death wasn't supposed to happen until later, after Angel turned back into the evil Angelus after losing his soul. Angelus was intended to kill him, then pursue a romantic relationship with Drusilla which would, in turn, break Buffy's heart.
Though Spike and Drusilla both became much more important characters on the show, it likely wasn't worth the effort to continue any sort of ongoing plot with the holiday, especially once Spike started his character arc into becoming one of the good guys. By season 7, he even sacrificed his life to save the world. It would have been easy to make a casual mention of the Night of Saint Vigeous at least once or twice throughout the remaining five seasons of Buffy the Vampire Slayer just to keep up with continuity, even in a sarcastic or backhanded mention, which was a staple of Joss Whedon's writing for the show. As there's no reason why vampires wouldn't continue celebrating the holiday, the lack of continuation there is a minor—albeit slightly disappointing—plot hole.