Some say it originated from the time of a terrible plague in Rome in 590 A.D. The Roman plague ended in spasms of sneezing or yawning. Gregory, as the new Bishop of Rome ordered that "God bless you" should be said to all who sneezed and the sign of the cross made on the mouths of those who yawned. It strikes me that the signing could have encouraged the plague's spread!
Today the Church of England remembers 'Gregory the Great, Bishop of Rome, Teacher, 604'. 3 September is the anniversary of Gregory's consecration in 590 A.D. as Bishop of Rome and 'Successor of St Peter'. Gregory (540 - 604 A.D.) was the first to refer to the role of 'Pope' as "servant of the servants of God" - a helpful image for any Christian minister. The English church (in all its expressions) has special reason to remember Gregory the Great - not for his instructions on sneezing or yawning, but because this 'Pope' sent a gifted Benedictine monk from Rome to our shores. This monk is better known as Augustine of Canterbury
The first children's history book I was given was 'Our Island Story' written in 1905 by Henrietta Marshall. It was a mixture of myth, legend and history of England from Roman times to the death of Queen Victoria. It had some wonderful pictures in it. The one I remember best shows Anglo-Saxon boys for sale in a slave market in Rome. This illustrated a traditional story recounted by Bede in his Historia Ecclesiastica Gentis Anglorum (History of the English Church, completed 731 A.D.) about how Gregory's concern first arose for the conversion of the anglo-saxon tribes in England. Here's a translation of what Bede wrote about Gregory:
"Among other merchandise Gregory saw some boys exposed for sale. These had fair complexions, fine-cut features, and fair hair. Looking at them with interest, he enquired what country and race they came from. 'They come from Britain,' he was told, 'where all the people have this appearance.' He then asked whether the people were Christians, or whether they were still ignorant heathens. 'They are pagans,' he was informed. 'Alas!' said Gregory, with a heartfelt sigh: 'how sad that such handsome folk are still in the grasp of the Author of darkness, and that faces of such beauty conceal minds ignorant of God's grace! What is the name of this race?' 'They are called angles,' he was told. 'That is appropriate,' he said, 'for they have angelic faces, and it is right that they should become fellow-heirs with the angels in heaven. And what is the name of thier Province?' 'Deira,' was the answer. 'Good. They shall indeed be de ira saved from wrath and called to the mercy of Christ. And what is the name of their king?' he asked. 'Aella', he was told. 'Then must Alleluia be sung to the praise of God our Creator in their land,' said Gregory, making play on the name. Approaching the Pople of the apostolic Roman see for he was not yet Pope himself Gregory begged him to send preachers of the the word to the english people in Britain to convert them to Christ, and declared his own eagerness to attempt the task should the Pope see fit to direct it. But this permission was not forthcoming, for although the Pope himself was willing, the citizens of Rome would not allow Gregory to go so far away from the city. But directly Gregory succeeded to the Papacyt himself, he put in hand this long cherished project and sent other missionaries in his place, assisting their work by his own prayers and encouragement."Well, true or not, it's a good story. You can read the rest of what Bede said about 'St Gregory the Great' here. Interestingly, in describing the mission to England Bede makes Gregory more important than Augustine of Canterbury who actually led the mission to Kent. I should say there were Christians in Britain long before Gregory sent Augustine - but that's another story.