surely harvest festival is a typical example of something that naturally happens if you're in a seasonal environment and practise agriculture (or even just gathering) - once you've collected in your food for the winter/year you celebrate it, everyone helps to accomplish the task and you need a sit down and a beer after all that hard work.
Likewise, Christmas or Yule or whatever - it's cold, it's miserable, it's time for a bonfire and another piss up before you all go insane in the long dark days.
now we're detatched from the land and the seasons, and can go down to Sainsbury's and get a punnet of strawberries in January