By the 1400s (for those of you who can't keep your centuries straight...better get that taken care of!) The Italian Renaissance and even the Northern Renaissance were well underway, and while your run-of-the-mill (haha) peasants still led quite medieval lifestyles, there was a lot of rebirth going on too! First, I think defining that century as Medieval has something to do with an Anglo-centric view. Since the 15th century English were still living in pools of cultural and actual feces (with a few major literary exceptions) while some of the continent flourished and rose from the muck, that somehow means everyone from that century gets to be confined to what people mistakenly think of as "the dark ages." The English are envious of other people's progress.
Another reason the 15th century comes up whenever someone wants to show something Medieval is because people are very unfamiliar with what actually happened in the incredibly long period we call the Middle Ages, and once you go back in time past, say, the 13th century, it just becomes mind-bogglingly long ago and quite scary and disgusting to think about. The 12th is perhaps my favorite medieval century (except for those pesky crusades), full of jousts and chivalry and no sign of a plague, and the 13th is medieval life at its high point -- you know, giant cathedrals, more crusades, increasingly overblown religion and all that. That is probably why people know so much about the 13th to 14th century (since those cathedrals took a century to build), and their customs, garb, etc. Even though I don't find those centuries all that exciting, they were the apparent high point before everyone died horrible pustule-ridden deaths and something completely different had to happen. (Also, people like Gothic stuff because it's pointy and vampiry, so they remember that High Middle Ages trend trend better than they remember the smoother, Romanesque cathedrals that predate Heaven-piercing spires.)
My pic from inside a Romanesque cathedral in Metz, France.
They are known for their Gothic one, but I thought this one was nicer!
They are known for their Gothic one, but I thought this one was nicer!
However familiar we are with those latter centuries of the Middle Ages, the myths and stories people most associate with the Medieval date back far earlier than that. Arthurian Legend in particular dates from the end of the Roman Empire! Think about it -- the fall of Rome led to the first Medieval Dark Age (which is an age without progress or much history being recorded -- the periods of the middle ages cannot all be referred to as "the dark ages" as people sometimes do), which lasted from about 500 to about 750 AD. Who the fuck knows what went on back then?! Besides a few students who might vaguely utter the word "Feudalism?" in a quiet, tentative upspeak, I think most of us don't know, and we don't want to know. Anyhow, Arthur, a legendary figure whom our Anglo culture exalts, didn't even speak English. He spoke some Celtic-Latin garbledygook, and actually defended Britain from those nasty Anglo-Saxons, the very blokes who would bring us our precious language! Yet there Arthur is on the Disney screen, speaking 1950s English, wearing 12th century clothes, and squiring for 12th century knights. The dude was, according to legend, king in the 6th century. That's like the year 500! Yikes. (Monty Python gets a little closer to the mark by putting Arthur in the 10th century, which is cool because it's well before 1066, and that makes a lot more sense. Ya'll English majors know what happened then, right? A bandmate of mine's address is 1066, and he wonders why I always say it resoundingly, "Tehhhhnnn Siiiixty-siiiiiix!")
Now Arthur, cathedrals, and all of that stuff is quite medieval even if it all happened in vastly removed centuries. I do wish it was more common for kids (and college students) to learn a little more about all the sub-periods of the Middle Ages, and to understand that many centuries in that long millennium were not very dark, but actually quite light with literacy, culture, and even technology (the metal plow by golly!). We spend gobs of time learning details about the Greeks and Romans, yet the next millennium gets skipped over and generalized. The millennium when our language came into its inchoate existence, the millennium when Catholicism and art did the same, the millennium when more and more (important) people started to read! It must have been an exciting millennium.
As exciting as the 15th century may have been, it was a century of returning to classical ideals, a rebirth of art and culture, and the beginnings of a flourishing economy in many areas. That is exciting of course, but it is not necessarily medieval. It's the end of an era and the beginning of something new. The full-blown "Renaissance" as we know it (which is problematic anyway because it started at different times in different places) makes it on the charts by the 15th century. That century was in fact transitional, and sometimes referred to by scholars as Late Middle Ages, but it certainly was not "all medieval and shit" the way some movies, books, teachers, and most kids think it was. If you want medieval, think the year 1000. Now that's medieval.
A note on "medieval":
Medieval means "middle age," as in the age in between when anything super-intelligent and progressive happened, I guess. Medi = middle. Eval = age. Please do not say (or worse, write in an essay) "the Medieval ages." You are in effect saying, "the middle ages ages" which really doesn't make a whole lot of sense. So that crazy place that calls itself "Medieval Times" is really saying "middle ages times." I try not to write "Middle Ages" because it's not as cool sounding as "medieval," and I kind of get a kick of of the fact that "medieval" is a lot harder to spell.
A note on capitalization:
Even folks like me with silly degrees still struggle with when to capitalize. This is particularly difficult when you are talking about a period. The name of the period, when used to refer to a period that has a loaded name like "The Enlightenment" or "Romanticism" is often capitalized, and it is also often capitalized when referring to work that is characteristic of that period, such as Enlightenment philosophy. Other words, however, never end up getting capitalized. We don't capitalize "existentialist," perhaps because it is not as tied to a period, but it is sort of a movement and I could see people wanting to capitalize it. Medieval is not often capitalized, probably because it is most often used as a very general adjective ("Look at that nerd in the medieval dress. Doesn't she know this is the Renaissance Faire?), and because the length of time it covers can't really be defined the same way shorter periods can. So while we have Romantic poetry, we also have medieval poetry. However, there is a need to capitalize medieval when you write on i.e. "Medieval England". In that case it's a big, loaded, proper name. When in doubt, the best thing to do is look for a handbook! (Or even Google it. I just tried typing in a few different "Do I capitalize...?" questions and it served up some decent sites.) In the end, some of it is preference, and you probably won't fail because of a capital "M" on the wrong type of "Medieval."
How to get schooled in the Medieval:
At college, it is covered in the first chunk of your usual freshman "Western Civilization" class, and it gets glossed over. But you can choose to take the class with a prof whose interests include medieval stuffs! Look up their bios, or just ask. I took it with a Chinese woman whose expertise was in early Medieval England. (I also took French with a Chinese woman. I think they must encourage all sorts of studies in the schools over there!)
As for medieval literature, If you are an English undergrad, there is usually a class that covers early English texts and spends a semester on that middle-millennium plus a little more, sometimes stopping right before or at Shakespeare. It will usually begin with Beowulf, which can be a treat at the college level. This class is also a hideous glossing over because, when you think about, it you read Old English, Middle English, and Elizabethan English all in one semester! Other survey courses at the undergraduate level usually only cover two or three centuries (which is still a hideous glossing over, but not quite as bad). I took "Anglo-Saxon to Renaissance Literature" as one of my period courses (I took three even though my school lamely only required ONE) after I'd taken the mostly medieval history class, and everything gelled wonderfully. I also took a "Medieval Manuscripts" class as a special topic, but that's just because I like smelly old books and I like visiting the Newberry Library's manuscript rooms.
I'm not a Medievalist nor do I plan to become one, but the stuff is quite fascinating once you learn a little more about it, and like I said, it's not all "dark." Oh yeah, did I mention the Middle Ages didn't start in the 15th century?