Harriet's visit of two months, starts in mid August, and it is the first week of October, or the last of September, when she tells Emma of the walnuts. Mr Martin got the nuts for Harriet in Autumn because the nuts would not be ripe in Summer.

The season that Emma has brought Mr Elton and Harriet together in, gleefully quoting MSND, is the antithesis of Midsummer. I think that the author is indicating that we might see a couple of seasons Emma has overlooked, in her eagerness to bring her match to fruition; that we can expect obstructions in the path of true love rather than fairy magic and immediate prosperity.