Lent then, like now, is a period of approximately 40 days, starting on Ash Wednesday and ending at Easter. (I shall talk in the past tense now even though some things are still relevant today) It was a time of penance and reflection so that people could prepare themselves for the celebration of the Resurrection. Forty days approximated to a tenth of the year (OK, not accurate, but close enough). A tenth was the amount that farmers gave to the church from the produce of their physical labour, and this was known as a tithe. Therefore the days of Lent were also known as Tithing days, when the faithful were expected to give a tenth of their year to God.
Lent was a time of fasting and prayer - or at least it was supposed to be. There were other fasting days too - the period of Advent, for example, and certain days of the week (Wednesday, Friday and Saturday) were allocated as days when no meat could be eaten. However Lent had stricter rules and must have been a pretty miserable time. For a start, as in the other fast days, no meat that came from either beast or fowl could be eaten. Fish however, because it came from the water and was therefore considered free of sin, could be partaken. Animal products, such as eggs, butter and milk were also banned; instead almond milk and olive oil was used in greater quantities than normal. And instead of the normal two main meals a day, there was supposed to only be one. You also need to bear in mind the time of the year - most of the stored fruit and vegetables were getting past their best and a great deal of flesh and fish left in the pantries were salted. Other activities were also meant to be curtailed during this time: music, dancing and sexual intercourse among them. However, as Edward III was conceived during Lent, obviously this was not always adhered to! Certain persons were absolved of fasting because of their physical state: the very old, sick, children and pregnant women. Even then they had to obtain a dispensation from the church. It should be noted that dispensations could also be bought, and this proved quite a lucrative time of year for the papal coffers!
Of course, for those who observed the no-meat rule, fish, fish and nothing but fish could get pretty boring. Cooks became quite creative with spices to try and zest things up a bit, and, to be fair, there was quite a variety of fish on offer: herring; mackerel; conger eel; plaice; salmon; shellfish; cod; lamprey; mullet; dory; turbot; barbel; dace; pike; fresh water eel; whiting; haddock and ling could all have been on the menu in the 14th century. Nevertheless, some did try and get round the meat ban by claiming that the tail of the beaver could be counted as a fish because the beaver lived in water. Likewise, it was argued that the barnacle goose could be eaten because it developed out of a barnacle and was therefore of the sea!
Snacking on food outside of the ‘official’ meal was also supposed not to happen. Luckily, ‘snacks’ did not include comfits (for example, dates in sugar), raisins, dates and sugar candy, so the rich, at least, could stave off some hunger pangs. But even with all the inventive ways people found to bypass some of the Lenten strictures, they must all have been glad when it ended - and that, of course, was with the plentiful feast (with lots of meat and eggs!) of Easter.
And just so you can see the sort of food that was eaten here are a couple of recipes suitable for Lent:
FOR TO MAKE A POTAGE OF RYS.
Tak Rys and les hem and wasch hem clene and seth hem tyl they breste and than lat hem kele and seth cast ther'to Almand mylk and colour it wyth safroun and boyle it and messe yt forth.
Take rice, sort it and wash it and boil it until the grains burst. Then let them cool. In the meantime set some almond milk to boil, colouring it with saffron. When it has boiled, serve it forth (one assumes on top of the rice). )
CRÈME OF ALMAUNDES.
Take Almaundes blaunched, grynde hem and drawe hem up thykke, set hem ouer the fyre & boile hem. set hem adoun and spryng  hem wicii Vyneger, cast hem abrode uppon a cloth and cast uppon hem sugur. whan it is colde gadre it togydre and leshe it in dysshes.
 spryng. sprinkle.
Take some blanched almonds, grind them up and add a little water. Put this over the fire and bring to the boil (I would imagine this would also need constant stirring). Take off the heat (when thickened), sprinkle with vinegar and lay the mixture on a cloth (this absorbs the excess liquid). Spread sugar on it. When it is cold, gather it together and slice it into dishes.)
Taken from The Forme of Cury, A Roll of Ancient English Cookery, compiled about A.D 1390, by the Master-Cooks of King Richard II. By Samuel Pegge (circa 1780) and downloaded fromProject Gutenberg.
If you are interested in medieval cookery I have a found a couple of good sites on the web:
Sources: The Forme of Cury (see above) Food and Feast in Medieval England by Peter Hammond (Sutton Publishing, 1993) Medieval Cookery: Recipes and History by Maggie Black (English Heritage, 2003) Living and Dining in Medieval Paris by Nicole Crossley-Holland (Cardiff University of Wales Press, 1996) The Great Household in Late Medieval England by C.M. Woolgar, (Yale University Press, 1999)