Im Wandel der Jahreszeiten (Through the Seasons) is a mini expansion that was available at Essen 2008 as a giveaway.
It's a postcard that shows the four seasons of a year aligned in a circle. Each season comes with an additional action field and some minor tweaks to the resource fields and some actions. The seasons are assigned to the rounds - one round, one season. You cycle through the seasons starting with a random one until the game ends.
Spring comes with an additional sow/breed action, summer with vacation (extra victory points), fall with harvest/vegetable and winter with family growth without space (for 2 wood and 3 food). The resource tweaks are usual +/- one wood/clay/reed/stone/fish and the action tweaks are usually freebies you get when taking the tweaked action (e.g. two free fences if taking fence action etc.).