Communism

In the field yes, but in and around house there was plenty of work. For example abovementioned threshing was often done in the winter, it was hellish work which took a lot of time up to even the next harvest. Also, what people often forget is that before the agricultural and industrial revolution technic and knowledge wasn't locked. XVIII and XIX century peasants, depite seemingly living in very similar conditions than their XII century ancestors still had better tools and more knowledge. They did had on average more hands but less land though, that was "recompensed" by serfdom and exploiting peasants from unpaid work.
Medieval peasant would simply be unable to work extra, that's why serfdom started to appear on wider scale in XV and XVI century after reneissance pushed knowledge forward and population completely recovered after Black Death - it was also beginning of rural capitalism formation, when commodity food production was increasing and of course solution was to squeeze peasant for more unpaid labour (for example, late XVI century Polish landowner could get up to 20 times more money from unpaid labour of peasants on his land than by taxing the same peasants from the same land).