This question touches several subjects. Let's address them separately.

First, the fasting during a Lent was more or less a necessity for large number of people with religious guidelines being derived from natural abundance of comestibles rather than imposed as an artificial regulation. The period when the Lent usually occurs, i.e. February and March (sometimes also April) is generally marked with very little food sources being available with the stockpiles being on the verge of depletion after the previous year's harvest. This means that people were forced to limit their consumption to make sure they will have enough to eat before the mid-spring when the first vegetables become available, give or take few weeks depending on yearly variances and climatic changes throughout Middle Ages.

Second, the agricultural production, unlike the manufacturing is natural rather than artificial and thus relies on the natural processes, not the actual human work that are needed only in specific periods with remaining effort being focused on relatively light ongoing maintenance. Thus, the actual 'production' of the agricultural goods (predominantly grain) is limited to the harvest periods when all the winter or spring grain is being collected at once. Of course, a lot of preparatory work needs to be done, including tilling, fertilizing and sowing that are generally labour-intensive endeavors. But it so happens, that the Lent period coincides with the time of the year when little actual field work is required as it typically starts only after Easter. The most common heavy labour associated with winter would be forest clearing, but again, this was not a regular task and its intensity would depend on the availability of the wood in the area. Either way, the lumber production would not be a part of agricultural production after all.

Third, the severity of restrictions were not uniform across Europe due to the differences in local cuisines and customs. It is suggested that the lent diet has been proposed by the early Christian philosophers who almost uniformly hailed from the Mediterranean region and thus used simple local diet as a model of moderate austerity. Fasting practices could have also been derived from the Aristotelian tradition of humour theory as land animal products (meat, cheese, eggs) were considered 'hot' and responsible for 'energetic' behaviors that also included boisterousness, anger, greed, lust and violence that are pretty much the opposite of humility, peace, austerity and self-restraint that was preached by the Church. One of the first link between the gluttony and lasciviousness can be found in the 4th century writings of Evagrius of Pontus that were reflected later in e.g. works of Thomas Aquinas. Thus, the common practice of limiting 'hot' comestibles (including alcohol) during many religious holidays and observations gave the basis for the common fasting practices.

This also meant that the average diet of an Italian or Provencal peasant was not that different from one encouraged during the Lent, but e.g. Germans, Poles or Burgundians would have faced serious problems due to completely different climate and availability of the products. Lent restriction usually called for an abstinence from meat of land animals and other animal products, such as eggs or butter. These was not problematic in the Mediterranean where fish and olive oil was abundant, but it was rather harsh restriction for inhabitants of northern European interiors for whom olive oil was usually unavailable and fishes could have been either expensive or limited to salted cods and herrings (also not too cheap due to transport costs). Of course, where is a will, there is a way and local clergymen, aware that Church's ideas of diet are far from universal, were allowing their parishioners to eat local products even if this was not that compliant with the letter of the canon law, sometimes unconditionally and sometimes (especially in late Middle Ages, after money became more abundant) in exchange for a small fee. On a side note, more affluent people could have always resorted to various tricks as their cooks, having access to many rare ingredients and spices could have prepared fishes that tasted almost like meat or made dairy equivalent of almonds, to name a few. Peasants were also trying to circumnavigate the regulations by using available plant oils from ubiquitous hemp and rapeseed, both for immediate consumption and to make butter or milk equivalents.

To sum it up, people in northern Europe were weaker during Lent and the late winter/early spring is a period generally associated with hunger regardless of religious observations, but as the required work was not as intense as in early winter or during the harvest, it was not really detrimental to the economy.