In comments many, John Lawler wrote:
Incidentally, Canterbury Tales is not written in English, so questions about its grammar are off-topic here. If this is not the original, but a translation, then it's the translator's dialect you are asking about.
Alas, that's not true, in many different ways. It was written in one dialect that some people used, of a language now dead, that no modern English speaker could understand without special study. The sentence you cite is clearly not a sentence the Chaucer wrote. It's a sentence in more modern English spelling, and more modern English syntax, than what Chaucer wrote. You should probably also note that Simple Wikipedia is not necessarily a good source of correct information.
It was originally written in an English language. There were a lot of them at the time, since most people never got more than 50 km from where they were born, and they'd all been speaking their own varieties of talk for hundreds of years like that. The language was called "Middle English"; what we speak is "Modern English". What Shakespeare wrote in was "Early Modern English", which is 200 years younger than Chaucer's Middle English.
But never mind who wrote it. The sentence in modern English is He was as fresh as the month of May (is), meaning the final is can be left off if desired. It's called an Equative construction, and the key to that is that it contains the "as ... as ..." correlative construction. That gives rise to lots of fixed comparisons: as fit as a fiddle, as strong as a horse, as smart as a whip, as dumb as a brick. So if the month of May is fresh, that's how fresh he is; but metaphors occasionally leave something to be desired. I don't really think of whips as smart, for instance.
Thou shouldst make free so to speak, an thou willst.