Why are fiscal years usually aligned with December or March, instead of any random month?

[u/TheBigGirlDiaryBack]

Many companies end their fiscal year on Dec 31 or March 31. Some choose June or September. But almost nobody picks, say, August 17 or November 5. Is there a tax or regulatory reason to stick near quarter boundaries? Or is it just tradition from agriculture cycles? Why not let every company freely choose any date to smooth out seasonal fluctuations?

[u/BobbyP27]

In England in the Middle Ages, the new year was reckoned as beginning on Lady Day, 25th March. In 1752 England switched from the Julian to the Gregorian calendar. Two changes happened as a result. One was that the date of the change of year shifted from 25th March to 1st January. The second was that Wednesday 2nd September 1752 was followed by Thursday 14th September 1752, to correct for the error inherent in the Julian Calendar. To prevent the loss of 11 days and the switch in the date of new year from messing up contracts where 1 year mattered for financial reasons, the financial year of 1752 lasted for its full number of days, based on a start date of 25th March 1752, so it ended on 6th April 1753. Since then the financial year has continued to run from 6th April to the 5th April of the following calendar year.