Capirotada is not the only special Mexican food associated with the Lenten season. Another is tortas de camarón, or shrimp fritters. These use dried shrimp, of the sort that can be found in markets that specialize in Mexican ingredients. If you find powdered shrimp, well and good; if not, grind up the whole tiny dried shrimp in a morcajete (mortar and pestle). With the tortas go red chile sauce and nopalitos (prickly pear pads)
These last can be purchased in jars or in the produce section of that same Mexican market where you bought the shrimp, or you can pick your own and prepare them. This latter takes a bit of care, so I advise you to look it up and follow the instructions.
Another dish that traditionally appears on the Lenten table is tortas de huevo, which one could describe as egg fritters. Beat egg yolks and whites separately (beating the whites until stiff), combine them with a little baking powder, and fry them up a tablespoon at a time. They are then topped with red chile sauce. For feeding the multitudes, some cooks layer the tortas and the chile sauce in a baking dish.
When I googled up these Lenten dishes, not only did I find several different versions of them, but in the comments that followed many of the recipes, people wrote in to explain how their family recipes differed from the ones they had just read. This is a pretty good indication that we are dealing with folk food that belongs in all of its variations to an entire community.
Of course, there is a whole battery of meatless dishes that can be, and are, served during Lent: Nopalitos in chile colorado, Sonoran enchiladas, chiles rellenos (with cheese), cheese enchiladas, strips of green chile sautéed with onion and perhaps some cheese, and the like. These are all part of the traditional local cuisine. But the three I’ve given special mention to are particularly associated with the Lenten season.
Fish is a traditional meat substitute all over the Catholic world. But getting fresh fish in inland Sonora during the years before reliable refrigerated transportation could be problematic. I have heard a story about an unspecified town in the sierras of Sonora. They were suffering a drought, and transportation from the coast was unreliable. The local priest and the municipal president jointly issued a decree to the effect that, for the duration of Lent, cows (or pigs, in another version) were fish!