If we go backwards in time using verifiable dates, what is the earliest event which can be associated with a specific day without question?
I am most interested in answers that actually show the chain of dates, not just an empty assertion (professor so and so said May 5, 2050 BC, is the earliest date...). For example, we know that the Inter gravissimas was issued on February 24, 1582, (old style) and that thereby October 4, 1582, was to be followed by October 15, 1582.
Therefore, given that in the Cronica Majora by Matthew of Paris it says that Harold Godwinson crowned himself king of England on the day that Edward was buried, which was "die Epiphaniae" (the day of the Epiphany, January 6th) in MLXVI Anno Domini. Since this was a date by the Roman calendar we can presume by the Gregorian calendar the event occurred exactly on the day 16 January 1066, 10 days later, assuming the Roman calendar used by Matthew was fully intact and continuous to 1582. Now, if you read any history book, such as the one cited by the Wikipedia, DeVries, K. (1999). "The Norwegian Invasion of England in 1066." (Woodbridge, UK: Boydell Press), it will say Harold became king on Jan 6, 1066, but we have just proved this is not true.
By this exercise we have shown that the dates you find in history books are often wrong. So, this brings us to the question again, going backwards from verifiable evidence what is the earliest event we can positively assign to a particular day. I have shown that we can assign the ascension of Harold to 16 January 1066, but I assume it is possible to go backwards much farther. The question is how much farther?
What is Meant by a Verifiable Date
By a verifiable date, I mean one where we can show absolutely the passage in time by days to the present day. For example, if the ascension of Harold occurred on January 16, 1066, and today is August 21, 2014, then we might think that the ascension of Harold took place 346461 days ago, or 948 years, 7 months, and 5 days ago. But is this really true? For example, if we consider that in 1066 the New Year occurred on April 1, not January 1st, then it is possible he ascended 949 years ago, not 948 years ago. An interesting question.
Aside from any calendar, how far back can we name the number of days to a particular event in history with confidence? Now, the person most expert at doing this was Joseph Scaliger, who invented the concept of the Julian Day for this exact purpose, and I am familar a little bit with some of the things he wrote, but since theoretically we have progressed in the last 200 years since Scaliger, I am thinking maybe someone has improved on his work. Hence the question.