If something happens very rarely you might say it occurs only once in a blue moon. But if you find yourself in a certain corner of northern England, someone might say such an event happens once in a Preston Guild.
Lancashire folk have a certain way with words, but in this case they are being literal. The Preston Guild only happens once every 20 years and is thus a fitting analogy for the rarest of rare things. So if you want to see something made special by its rarity, make your way to Preston because 2012 is a Guild year, and Preston aims to take its place amidst the national Jubilympic celebrations.