The equinoxes might be expected to be in the middle of their respective seasons, but temperature lag (caused by the thermal latency of the ground and sea) means that seasons appear later than dates calculated from a purely astronomical perspective. The actual lag varies with region, so some cultures regard the autumnal equinox as "mid-autumn" whilst others with a longer lag treat it as the start of autumn.Meteorologists (and most of the temperate countries in the southern hemisphere) use a definition based on months, with autumn being September, October and November in the northern hemisphere, and March, April and May in the southern hemisphere.
In North America, autumn is usually considered to start with the September equinox. In traditional East Asian solar term, Autumn starts on or around 8 August and ends on about 7 November. In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September, October and November.However, according to the Irish Calendar which is based on ancient Celtic traditions, autumn lasts throughout the months of August, September, and October, or possibly a few days later, depending on tradition. In Australia, autumn officially begins on 1 March and ends 31 May. The vast diversity of the ecological zones of the South American, African and Australian continents renders the rigid European, North Asian and North American seasonal calendar an imposed cultural concept rather than relevant to climactic conditions. The seasonal cycles as named and described by the various indigenous Aboriginal peoples of Australia differ substantially from one another according to their local geographical and ecological environment and are intricately dependent on local environmental events and resources.
source : wikipedia.com