Just found this thread - had I done so earlier I could have answered some of the questions being posed, e.g. It is April not Avril, but pronounced in the Borders as Appril and the kale plants are indeed from Hawick.
The song was written by David Anderson (NB NOT Cumberland dialect poet Robert Anderson), who was what English literature academics like to call a 'labouring class poet' from Newcastleton - or Copshawholm/Copshawholme, the old name of the lands were called on which the Duke of Buccleuch built the planned village of Newcastleton or Castleton in 1793. David Anderson published 'Copshawholm Fair' in his poetry collection Musings by the Burns and Braes of Liddesdale (C. Thurnam & Sons, Carlisle, 1868), in which he adds the note that says he composed the song some 38 years before, i.e. 1830, and notes that the hiring fair was an annual event. In fact, according to an 1880s Gazetteer hiring fairs were held here three times a year: second Friday of April, and Fridays before 17 May and 8 November. Another source says the last hiring fair was held in 1912.
Newcastleton is also, of course, home to a fine Traditional Music Festival each year - 2017 will be the 48th festival and runs 30 June - 2 July.
Anderson's words are copied below:
by David Anderson
 On a Friday it fell, in the month of April,
Ower the hills cam' the morn, wi' her blithesomest smile,
The folk were a' thranging the roads everywhere,
Making haste to be in at the Copshawholm Fair.
 They are seen coming in frae mountain and glen,
Baith rosy-faced lasses and strappan young men,
A' jumping wi' joy and unburdened wi' care,
When meeting auld frien's at Copshawholm Fair.
 'Tis a day when old courtships are aften renewed,
Old disputes set aside or more hotly pursued;
What Barleycorn Johnny sees fit to declare
Is law—for he's king at the Copshawholm Fair.
 There are lads for the lasses and toys for the bairns,
And blind ballad-singers and folk wi' no airms;
A fiddler is here, and a thimbler is there,
Wi' nut-men and spice-men at Copshawholm Fair.
 There's pethers and potters, and gingerbread stan's,
Peep-shows, puff-and-darts, and the great caravans;
There's fruit frae all nations exhibited there,
And kail-plants frae Hawick at Copshawholm Fair.
 Now next 'bout the hiring, if you want to hear tell,
You shall ken it as far as I've seen it mysel';
But what wages are gien it is ill to declare,
Sae muckle they vary at Copshawholm Fair.
 Only ane I saw hired, a strappan young queen,
Heard her spier'd what her age was, and where she had been,
What work she'd been doing, how long she'd been there,
What wages she wanted at Copshawholm Fair.
 At first the young lassie a wee while stood dumb,
She blush'd and she scrapit wi' her foot on the grun';
At last she took heart, and did stoutly declare—
I'll hae five punds and ten at Copshawholm Fair.
 Says he, but my lass, that's a very big wage,
And turning about as he'd been in a rage,
Says, I'll gie thee five punds, but I'll gie nae mair,
I think thou maun tak' it this Copshawholm Fair.
 He held out the shilling to arle the bit wench,
In case it should enter her noddle to flinch;
She grap at it, muttering, I should hae had mair,
But yet I will tak' it at Copshawholm Fair.
 Now the hiring is done, and off they a' spang,
They rin to the ball-room to join wi' the thrang;
"I never will lie wi' my mammy nae mair"
The fiddle plays briskly at Copshawholm Fair.
 There is one in the corner sits drinking his gill,
Another beside him sits sipping his yill,
Another is strippit, and swearing right sair,
Room, will ye no' gie me at Copshawholm Fair?
 Now this is the fashion; they thus pass the day,
Till night comes at last and they ellie away;
But some are so sick that they canna do mair,
With dancing and fighting at Copshawholm Fair.
[Formatting fixed by a Mudelf.]