During the Middle Ages, Michaelmas was celebrated as a Holy Day of Obligation.
It was also one of the English, Welsh and Irish quarter days when accounts had to be settled. On manors, it was the day when a reeve was elected from the peasants. On the Isle of Skye, Scotland, aprocession was held.
http://www.historic-uk.com/CultureUK/Michaelmas/
Folklore in the British Isles suggests that Michaelmas day is the last day that blackberries can be picked. It is said that when St. Michael expelled Lucifer, the devil, from heaven, he fell from the skies and landed in a prickly blackberry bush. Satan cursed the fruit, scorched them with his fiery breath, and stamped and spat on them, so that they would be unfit for eating. A Traditional Irish proverb says:
On Michaelmas Day the devil puts his foot on the blackberries. If you have access to blackberries, make this the last picking and eating. Perhaps make a blackberry pie?
The act of giving a Michaelmas Daisy symbolises saying farewell, perhaps in the same way as Michaelmas Day is seen to say farewell to the productive year and welcome in the new cycle.
Traditionally, in the British Isles, a well fattened goose, fed on the stubble from the fields after the harvest, is eaten to protect against financial need in the family for the next year; and as the saying goes:
“Eat a goose on Michaelmas Day,
Want not for money all the year."
In Christianity, the Archangel Michael is the greatest of all the Archangels and is honoured for defeating Lucifer in the war in heaven.[2] He is one of the principal angelic warriors, seen as a protector against the dark of night, and the administrator of cosmic intelligence. Michaelmas has also delineated time and seasons for secular purposes as well, particularly in the United Kingdom and Ireland as one of the quarter days.
St Michael is also the patron saint of horses and horsemen. This could explain one of the ancient Scottish traditions that used to be practiced on Michaelmas Day. Horse racing competitions in the local communities would be held and small prizes won. However, with a twist, it was the only time at which a neighbour’s horse could be taken lawfully the night before and ridden for the entirety of the day, as long as the animal was returned safely!
The Eastern Orthodox Churches do not observe Michaelmas. The Greek Orthodox honour the archangels on 8 November instead.

HolyScripture describes St. Michael as "one of the chief princes," and leader of the forces of heaven in their triumph over the powers of hell. He has been especially honored and invoked as patron and protector by the Church from the time of the Apostles.
Although he is always called "the Archangel," the Greek Fathers and many others place him over all theangels - as Prince of the Seraphim. St. Michael is the patron of grocers, mariners, paratroopers, police and sickness.
Saint Michael the Archangel,
Defend us in battle.
Be our protection against the
Wickedness and snares of the devil.
May God rebuke him,
We humbly pray.
And do thou,
O Prince of the Heavenly Host,
By the Power of God,
Cast into Hell
Satan and all the evil spirits
which prowl about the world,
Seeking the ruin of souls.
Amen.
Facts:
Feastday: September 29
http://proecclesia.blogspot.ca/2011/09/michaelmas-feast-of-st-michael-and-all.html
Patron of grocers, mariners, paratroopers, police, and sickness
http://www.2heartsnetwork.org/Michael.htm