Comment by antisthenes

I think the origin of the trope is that the peasants sat around doing nothing during winter, when there was nothing to plant or harvest.

It's probably true that there was less work in the winter (although you still had all your maintenance tasks, e.g. repairs and preparing firewood), but this was compensated by much more intense labor in the spring and summer.

Overall, though, it makes no sense to say medieval peasants worked less than people do now, it's likely very comparable, and the variations would depend on the quality of your soil/irrigation and how much you were going to get taxed.