Having had some success with changing our diet a bit in order to get a sense of the medieval European diet, this year Cindy and I decided to skip meats for lent.
Many modern Catholics eat fish instead of (terrestrial) meat on Fridays during lent. During the medieval period though, the common practice was much more restrictive. Aside from meats, dairy and eggs were also off the menu. There were some typical substitutions for the wealthy - almond milk, almond cream, almond butter - but for the most part it was nothing but fish and plants for 40 days.
The reasons for these restrictions (other than theological) are unclear. I've heard that at this time of year poultry would have been laying few eggs, so not eating eggs makes sense. Also, I assume that any animals that one didn't intend to keep through the winter would have already been slaughtered in the late autumn, so not eating meat also makes sense. But dairy?
I suppose (caveat lector: I am not a dairy farmer) that milking cows over winter when there is limited feed would stress them further and reduce their chances of reaching spring in a healthy state. By not milking them they'd require less fodder, and even though they'd dry up, when they calved in the spring the milk would start flowing again.
At any rate, we'll be splitting the difference between the modern and medieval Lenten diet. No terrestrial meats on any day, but I'm granting us an indulgence for dairy and eggs. I don't expect it'll be too difficult for us given that we were primarily vegetarian for a couple of years a long while back, but for our children it's a new experience (especially for Alex, who often says things like "Animals are yummy!"). Next year maybe we'll go completely medieval.