10 .
Harvest Festival (as such) is, perhaps, a relatively recent addition to the calendar of many churches, having been instituted in the 19th century ... not all that long, within a church-historical perspective of 2,000-odd years, before Remembrance came in after World War 1. Indeed, for churchgoers (and active organists like your author), Harvest in the early autumn is usually followed fairly closely by Remembrance in early November as an occasion for a special service.
It's all very well thanking God for all the bounties of the harvest ... but what would have been the 'other end of the arc': the Sunday when Anglicans, at least, pray to God around the (apparent!) START of the farming year?
Candlemas
Michaelmas
Rogation Sunday
Lady Day