The old Icelandic calendar counted the year not in months but in weeks, splitting it into two misseri, summer and winter, of twenty six weeks each, three hundred sixty four days total, a structure that fits neatly into weeks but falls short of the true solar year by roughly a day and a quarter every year. Left uncorrected, that shortfall would have pulled the start of summer steadily backward through the actual seasons within a single generation.
Islendingabok, Ari Thorgilsson's twelfth century Book of the Icelanders, credits the fix, sumarauki, an extra week inserted into summer roughly every five or six years, to the lawspeaker Thorsteinn Surt, adopted at the Althing, Iceland's assembly. A fuller layer of named months, Gormanuour opening the winter half, Thorri and Goa in its depths, and others through summer, developed on top of the two misseri, week counted skeleton, giving the calendar both a coarse legal structure, built from misseri and weeks, and a finer named month structure used more informally, operating together rather than as competing systems. Because the calendar was defined by week counts rather than by a leap day tucked into a particular month, its correction mechanism was structurally distinct from the Julian or Gregorian solution to the same underlying problem, a genuinely different engineering answer to keeping a year in step with the sun.