> The vast majority people in the Middle Ages lived off subsistence farming, which doesn't really have "days off".

I think you have an incorrect view of subsistence farming. It does not require constant work, and you do not see constant work in traditional societies. You do see many days off. Now during harvest time there is lots of work, but that is a small period of the year, say 7 weeks out of the year. Other times the farmer has the option of when to plant and how much, when to weed and how much, when to work on processing food and how much. And these fall into familiar patterns from which things like feast days emerge as the interstitial times when the above does not need to happen. The idea of constant backbreaking labor all year long may be appropriate for something like a large Jamaican plantation, but not a small farm in Europe. Of course if it was an American farmer, they would enlarge their farm, bring in more livestock or auxiliary sources of income to fill up whatever daylight was available, and keep adding work until they and all their hired hands were busy all the time. Then they would take the earnings and use it to buy more land and hire more people and expand the work even further. But traditional societies did not organize themselves this way, nor is this something required of farmers.

Here is some data:
http://groups.csail.mit.edu/mac/users/rauch/worktime/hours_w...

"The contrast between capitalist and precapitalist work patterns is most striking in respect to the working year. The medieval calendar was filled with holidays. Official -- that is, church -- holidays included not only long "vacations" at Christmas, Easter, and midsummer but also numerous saints' andrest days. These were spent both in sober churchgoing and in feasting, drinking and merrymaking. In addition to official celebrations, there were often weeks' worth of ales -- to mark important life events (bride ales or wake ales) as well as less momentous occasions (scot ale, lamb ale, and hock ale). All told, holiday leisure time in medieval England took up probably about one-third of the year. And the English were apparently working harder than their neighbors. The ancien règime in France is reported to have guaranteed fifty-two Sundays, ninety rest days, and thirty-eight holidays. In Spain, travelers noted that holidays totaled five months per year.[5]

The peasant's free time extended beyond officially sanctioned holidays. There is considerable evidence of what economists call the backward-bending supply curve of labor -- the idea that when wages rise, workers supply less labor. During one period of unusually high wages (the late fourteenth century), many laborers refused to work "by the year or the half year or by any of the usual terms but only by the day." And they worked only as many days as were necessary to earn their customary income -- which in this case amounted to about 120 days a year, for a probable total of only 1,440 hours annually (this estimate assumes a 12-hour day because the days worked were probably during spring, summer and fall). A thirteenth-century estime finds that whole peasant families did not put in more than 150 days per year on their land. Manorial records from fourteenth-century England indicate an extremely short working year -- 175 days -- for servile laborers. Later evidence for farmer-miners, a group with control over their worktime, indicates they worked only 180 days a year."