Why was one of the "worst days" come in SEPT. What happens?

From the text:

One of the worst days we had in the whole time, as I thought, was in the beginning of September, when, indeed, good people began to think that God was resolved to make a full end of the people in this miserable city. This was at that time when the plague was fully come into the eastern parishes. The parish of Aldgate, if I may give my opinion, buried above a thousand a week for two weeks, though the bills did not say so many;—but it surrounded me at so dismal a rate that there was not a house in twenty uninfected in the Minories, in Houndsditch, and in those parts of Aldgate parish about the Butcher Row and the alleys over against me. I say, in those places death reigned in every corner. Whitechappel parish was in the same condition, and though much less than the parish I lived in, yet buried near 600 a week by the bills, and in my opinion near twice as many. Whole families, and indeed whole streets of families, were swept away together; insomuch that it was frequent for neighbours to call to the bellman to go to such-and-such houses and fetch out the people, for that they were all dead.