I think America (maybe Canada too) is the only country where autumn (no capital A!) is deemed to begin on 22nd/23rd September. The rest of us think ye're a bit weird for doing that.
stiggie has it right - the equinox (and the solstice) mark the midpoint of the seasons - hence why Midsummer festivals are in and around the 21st of June.
The differences come down to when you define the seasons - which are purely arbitrary to start with. If you define, say, summer as the hottest three months of the year, this I think does tend to be from 23rd June to 23rd September in general.
But if you define summer as the three months with the most daylight, then that's the three months with the solstice in the middle (i.e. May/June/July). Christmas is on the 25th December because it took over from an old pagan festival celebrating the passing of the mid-point of winter - 21st December - after which the light starts to come back and you could start to see the shift back to the easier life of spring/summer.
I don't know why Europe then changed summer to be June/July/August, leaving Ireland on its own as having summer in May/June/July, but there you go. The Irish for September is Me?n F?mhar - literally "The middle of autumn", and the Irish for October is "Deireadh Fomh?r" - literally "The end of autumn". The four main pagan Irish festivals also celebrate the new seasons - Bealtaine on 1st May (first day of summer), L?ghnasa on 1st August (first day of autumn), Samhain on 1st November (first day of winter - this morphed into Hallowe'en) and Imbolc on 1st February (first day of spring - which the Catholic Church turned into the feast of St Brigid, which it still is, meaning that Imbolc is very much the forgotten pagan festival)
None of which helps explain when CHM are back on the screen though.