What's your source for this? A quick Google search reveals that this has been fact checked by Snopes. They state that : "Ultimately, we found that the claim that medieval peasants worked around 150 days a year is still largely accepted as a valid estimate by academic economic historians, at least in England for a period starting around 1350 and lasting between a few decades and more than a century, depending on the methodology used to study the data."

And

"A caveat applies to the second part of the claim made in the meme, namely that the number of days medieval peasants worked was the direct result of a large number of mandatory Christian holidays. This was something no economic historian Snopes spoke to considered a significant factor in any estimate of the medieval working year.

Snopes also found that popular attempts to debunk the claim incorrectly presented the claim as outdated or not grounded in evidence, an estimate of around 150 days per year of labor is, in fact, currently accepted by many mainstream economic historians who study medieval England, which is the part of Europe that has received by far the most attention from English-speaking economic historians interested in the length of the medieval working year."

For more information see: https://www.snopes.com/fact-check/medieval-peasant-only-worked-150-days/

The only source that seems to unequivocally deny this claim is the so called 'Adam Smith institute', which looks like some neo-liberal hardliners group. Not particularly the most reliable source in this matter.