It’s that day. In the rhyme as I remember it,
Remember, remember, the fifth of November:
Gunpowder, treason, and plot!
And from this event, in 1605, we get the colloquial noun guy ‘man’.
Other sources have the second line:
The Gunpowder Treason and plot;
and the continuation:
I see of no reason why (the) Gunpowder Treason
Should ever be forgot.
In any case, the traditional rhyme is dactylic tetrameter, with some metric variation in the lines, including a rest (a beat with no text filling it) at the end of lines 2 and 4. My recollection is that the rest in line two is filled by a firecracker: Gunpowder, treason, and plot! BANG!
On Guy Fawkes the man, from Wikipedia:
Guy Fawkes (13 April 1570 – 31 January 1606), also known as Guido Fawkes, the name he adopted while fighting for the Spanish in the Low Countries, was a member of a group of provincial English Catholics who planned the failed Gunpowder Plot of 1605.
… Fawkes became synonymous with the Gunpowder Plot, the failure of which has been commemorated in England since 5 November 1605. His effigy is traditionally burned on a bonfire, commonly accompanied by a firework display.
That brought us Guy Fawkes’ Day (for a long time, a deeply anti-Catholic event), with Bonfire Night or (later) Cracker Night, from cracker ‘firecracker’. More recently, celebration of American-style Halloween has apparently been threatening to swallow up Guy Fawkes’ Day.
Then, the development of guy, from OED2:
1… An effigy of Guy Fawkes traditionally burnt on the evening of November the Fifth, usu. with a display of fireworks. Also in full Guy Fawkes.
Guys were formerly paraded about in the streets on the anniversary of the ‘Gunpowder Plot’ (Nov. 5). They are now more frequently exhibited by children collecting money for fireworks during the days preceding Nov. 5. [“a penny for the guy”]
The figure is habited in grotesquely ragged and ill-assorted garments (whence sense 2), and was formerly accompanied by other similar effigies (representing unpopular persons), to which the name of ‘guys’ is often given by extension.
1806 W. Burrell in Lett. C. K. Sharpe (1888) I. 277 A month ago there was neither shape nor make in me… No guy ever matched me.
2. A person of grotesque appearance, esp. with reference to dress; a ‘fright’. [1836 on]
3… A man, fellow. orig. U.S.
The earliest examples may be influenced by sense 2. [1847 on] [colloquial guy here falls in a set of terms including fellow, lad, get, chap, dude, joe, and hombre]
Draft additions Oct. 2011: colloq. As a form of address to a man (cf. sense 3d). Also in pl. as a form of address to a group of people, in later use sometimes a mixed or all-female group. [1876 on]
There are some puzzles here that need to be filled in by a proper cultural historian, something I am not. One: that’s two centuries from the original Guy Fawkes event to the OED‘s first cite for guy referring to the ragged effigy; what was going on during this period? Two: what’s the history that leads from the ‘person of grotesque appearance’ sense (a straightforward extension from the ragged-effigy sense), in Britain, to the development of an ameliorated, more general, sense in the U.S. specifically?
As a side matter, I wonder what a penny for the guy in 1806 would translate into in today’s money.