We now think of three meals a day as the norm. Everybody knows about breakfast-lunch-dinner, though the fast food industry has tried to sneak a "fourth meal" in, pizza-time, coming around 9 or 10 at night. And of course there's British tea-time, which can come in late afternoon and may substitute for the evening meal.
But this was not the medieval norm. In the morning, you got up and got to work. No cereal and orange juice, no coffee or tea, no bacon and eggs, no blueberry pancakes. At most one would grab a piece of bread and a mug of beer.
Now "morning" of course began at sun-up, which was, by definition, 6am. By our standards, 6am moved around a lot, coming very early in midsummer, late in midwinter. But this did not worry medieval people.
One got the day's work in, then had "dinner" at "noon." Now noon did not come in the middle of the day, as we think of it. The word comes from the Latin "none," the ninth hour. Because sun-up was, by definition, 6am, and sunset 6pm. the ninth hour was the middle of the afternoon, what we'd call 3pm at the equinox. This was dinner time, the one big meal of the day.
(I do believe that our use of the term "noon" for midday is the result of trying to move dinner-time earlier and earlier. Who wants to wait until 3pm?)
During the summer, of course, one might work what they'd call 9 hours and we'd call 12 hours or more before dinner time, so there might be a snack in there, but not a meal, at most another piece of bread and pull of beer. Monks, whose daily work consisted of prayer and singing the psalms and reading and copying manuscripts, were warned against the "midday demon" of hunger, because they didn't get snacks, and the midday demon would make them think about food rather than what they were supposed to be thinking about.
Our word "dinner" comes from the French "déjeuner," to break one's fast, that is to end the long period (perhaps close to 24 hours) in which one had not eaten. In American usage, the word "dinner" still means the big meal of the day (whether at 12 or 6), even if in the modern US we assume that we've broken our fast hours earlier with "breakfast." The French still have "déjeuner" at midday, preceded early in the morning by "petite déjeuner," the little breaking-of-fast, which while consisting of coffee and a roll rather than a mug of beer, is still pretty minimalist by American standards.
(In the last few decades, however, the French have been increasing the size of the "pj" to include juice, granola, yogurt, cheese, and ham, though still not anything hot beyond the coffee.)
After the big afternoon meal, medieval people would have their more relaxing part of the day. The one big meal might hold them, or they might have a little soup for "supper" ("souper" in French) before turning in.
In the modern US, there are still familial and regional differences as to whether the evening meal should be called "supper" or "dinner," with some insisting that a "dinner" is a big noon meal, not an evening meal. Mediterranean countries and France still tend to have the biggest meal of the day at midday. The French refer to the evening meal as "dîner," which of course is from the same root as "déjeuner," but means something different. In modern French, "souper" is what we'd call an evening snack, what one might have after an performance at the theatre before heading home. In the twenty-first century, it is sometimes even pizza.
Click here and here for more on what medieval people ate and did not eat.