The useful thing about seasonal references is that not only can they pin down the season as a whole but narrow it down. In this case we have the season narrowed down to a day. We may not know which year, but unfortunately, even if the author forgets, one or both of the ex-partners will remember.
Autumn is a poignant time in haiku poetry, and one for reflection. I feel that one or both of the ex-partners isn’t into the friendliest of reflections.