To get a better understanding of this, one must realize that in the Middle Ages people more willingly accepted tradition as being true. Pious traditions or legends were told from time immemorial were simply believed.
It seems that the number one source of your pious legends comes from the Golden Legend by Blessed Jacobus de Voragine (1230-1298).
Blessed Jacobus de Varagine or Voragine (Italian: Giacomo da Varazze, Jacopo da Varazze; c. 1230 – July 13 or July 16, 1298) was an Italian chronicler and archbishop of Genoa. He was the author, or more accurately the compiler, of Legenda Aurea, the Golden Legend, a collection of the legendary lives of the greater saints of the medieval church that was one of the most popular religious works of the Middle Ages. - Wikipedia
March 25 according to the Golden Legend is rich in symbolism:
According to this legend, the Creation of Adam, the Fall of Adam, the murder of Abel, Melchisedech's offering of bread and wine, Abraham's near sacrifice of Isaac, the Annunciation to Mary and Conception of Christ, the decapitation of John the Baptist, the Crucifixion of Christ, James' killing and Peter's escape all occurred on March 25th. - Davidlol
Please feel free to peruse the entire Golden Legend when and if you have the time to do so from the Medieval Sourcebook: The Golden Legend (Aurea Legenda)
Compiled by Jacobus de Voragine, 1275 Englished by William Caxton, 1483.
Some “Historical” Dates in Medieval Liturgical Calendars from England may be of interest to you even though some of these dates are not the same, but are relatively close. For example, the Creation of Adam was on March 23.
He also notes the dating of the creation to 25 March as well:
The symbolism attached to the Julian date of the spring equinox was not limited to the Passion or Resurrection of Jesus. The Alexandrian monk Annianos, for instance, dated the creation of the world and the incarnation (or conception) of Christ on 25 March as well. The vernal equinox was associated with the creation as early as the third century, but opinions differed on the day of the week of the creation on which 25 March had to be placed: some preferred the first day (Sunday), when God separated light from darkness, others decided in favour of the fourth day (Wednesday), on which God created the so-called luminaries, i.e., the sun, the moon and the stars. ...
The idea that Jesus was born (or conceived) and crucified on the same calendar day, and thus lived a perfect number of years, is first attested in the Easter table of Hippolytus of Rome (AD 222). - “Historical” Dates in Medieval Liturgical Calendars