I found a web page that describes why that particular date was chosen. Although the term Julian Calendar derives from the name of Julius Caesar, the term Julian day number probably does not. Perhaps it was simply named after the Julian Calendar. Astronomers, as noted above, preferred to use .00 to mean midday and .50 to mean midnight. This was not to the liking of all scholars using the Julian day number system, in particular, historians.
Days begin at midnight rather than noon.
For dates in the period from 1859 to about 2130 only five digits need to be used to specify the date rather than seven.