This is adapted from Wikipedia.

In 2011, the Welsh author Roger Bryan discovered this poem written at the bottom of a page of saints' days within a Latin manuscript in the British Library's Harleian manuscripts. He dated the entry to 1425 ±20 years.

Thirti dayes hath Novembir
April June and Septembir.
Of xxviij is but oon
And alle the remenaunt xxx and j

Seems like they're rhyming the word 'one' with itself ('j').

Note that November is taking the place that September does in the modern rhyme. The early versions tended to favour November and as late as 1891 it was being given as the more common form of the rhyme in some parts of the United States.

The unhelpfulness of such an involved mnemonic has been mocked, as in the early-20th-century parody "Thirty days hath September / But all the rest I can't remember."