This question is usually about Pennywise/IT from Stephen King’s novel It , which returns to Derry roughly every 27 years.
In‑story reason
Within the story, IT is an ancient, otherworldly predator that feeds on fear, especially children’s fear, then goes into a long hibernation to recover. The 27‑year gap is presented as its natural feeding cycle: it wakes for about a year or so, feeds, then sleeps again beneath Derry until it has regained enough strength to return.
Why 27 specifically
The exact reason for the number 27 is never fully explained in the novel or films; it is treated as part of IT’s supernatural nature and connection to cosmic forces. Some interpretations suggest it reflects a “generational” rhythm—roughly the time for one group of children to grow up and a new set of vulnerable kids to take their place—so the creature always has fresh victims whose parents have half‑forgotten the last cycle.