'Banal' is a common word these days, and it means 'common'. It was not always so common, and has a rather curious history. Most people use the word, but many will be uncertain whether to say BAY-nal or ba-NAHL or even BAN-al or ba-NAL. To answer this requires a look at the history.

I suppose it's usually pronounced ba-NAHL, as if it's French: and so it's like locale, morale, and rationale, which in English we also treat as French, and spell with an extra -e on the end in the belief that that makes it look more French. I haven't seen the spelling banale yet, but I'm sure it'll turn up. A small dictionary will even tell you that 'banal' is from French. It is, but this is only half the story.

The word 'banal' derives from the very old word 'ban'. The original meaning of 'ban' is just 'announcement', something spoken. Thence (i) a ban as a royal or feudal proclamation; and (ii) the banns announcing a marriage. From sense (i) came the modern, more specialized sense of a proclamation forbidding or outlawing something. But it also gave the specific meaning of a command by a feudal lord that all the people must use his mill, and presumably pay him for the privilege. This feudal service contained in a ban or announcement was therefore called banal service. Something was banal if everyone had to do it: it was common to all.

So it's an old word, and this justifies the pronunciation BAN-al, which I admit is now never heard. The alternative BAY-nal, which used to be the usual pronunciation, is now also in a minority. People have switched to ba-NAHL in the idea that it's a modern French borrowing. Of course most adjectives ending in -al were originally borrowed from French, and ultimately from Latin: casual, national, vital, tonal, personal, etc etc. But they've been firmly established in English for almost a thousand years, and are native English words as far as pronunciation goes. 'Banal' ought to be too. It was in fact taken from French later than the others, originally with the spelling 'bannal', which tells you how the first borrowers pronounced it. But its connexion with the feudal ban has been lost, and now that it's gained in popularity, become rather a cliché, people don't know what it relates to, and have reinterpreted it.

Before I knew this, I used to guess it came from bain 'bath', and meant 'numbing' the way a warm bath does. There is no obvious reason why the pronunciation ba-NAHL should arise. Wrong reinterpretations of unfamiliar words do occur. Another example is 'jejune', which is straightforwardly from a Latin word meaning 'fasting': thus 'thin, insipid'. But this Latin jejunus is unknown these days, while French jeune 'young' is well-known, so perhaps 'jejune' is really 'young, unformed, green'? You occasionally see the spelling 'jejeune'; and it's even been seen equipped with accents and italics to indicate the (imaginary) Frenchness: jéjeune. If you ever see the spelling banale you'll know it's succumbed to the same kind of misinterpretation. (To add to the confusion, the French for 'jejune' is in fact jeûne, unrelated to jeune.)

Actually, looking again at the OED, what I think happened is that it was borrowed twice from French, first in the 1700s as 'bannal', referring to the feudal duty, and then in the early 1800s as banal in the modern sense (presumably in French) of 'ordinary, tiresome, clich├ęd'. So it was this reborrowing that justifies the ba-NAHL pronunciation which is now dominant.

Jabberwocky's sense of 'ultramundane' in the preceding write-up is new to me. To me, 'banal' is an ordinary word meaning 'commonplace, trite, dull, trivial, platitudinous'. What seems to have happened in the science-fiction usage described above is that it's been used as a synonym for 'mundane', which it is, but then specifically given the special connotations of 'mundane', viz 'worldly, of this world, of the ordinary world' (Latin mundus = 'world'); traditionally the mundane contrasts with the heavenly, if you're going to focus on which 'world' it applies to: by a natural extention some modern authors have contrasted it with a different world. Then this contrast has been switched from 'mundane' onto its near-synonym 'banal'.

To go back to origins, and meet some distant cousins. 'Ban' originally meant announcement: it's from the Indo-European root BHA- meaning 'speak'. This is a very widespread root. Other Germanic forms include banish and boon and bandit (= proclaimed outlaw). The Greek form is PHO- or PHA- or PHE-, as in phonetic, telephone, and euphemism. The Latin is FA-, as in fable (= speak-able) and fate (= spoken) and infant (= un-speak-ing) and fairy (Faerie = place of Fates) and fame. Not such a banal word after all.

You'll notice nowhere do I recommend how you should pronounce it. That's not my job: I'm just laying out facts.