The Gregorian Calendar
Over the centuries, by following the Julian calendar, the Easter festival was slowly but surely moving away from the spring equinox towards the summer solstice. The new system adopted by Pope Gregory (the Gregorian Calendar) in the 16th century, specified a calendar with a year length of 365 days, 5 hours, 40 minutes, 20 seconds. This required 3 days to be dropped every 400 years. So those years which were divisible by 100 would only be leap years if they were also divisible by 400. To correct errors which had built up over the centuries Pope Gregory declared that Thursday 4th October 1582 in the Julian Calendar should be immediately followed by Friday 15th October in the Gregorian Calendar (thus adding 11 days).
The date of Michaelmas, being an ecclesiastical feast day stayed at September 29th, but the date of the Shefford Fair moved forward 11 days to the 10th October as it is currently celebrated.