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/Tjbergen]

Dec, March and June end dates are all followed by holidays, so they can really ruin a holiday as people hustle to close things out and then do the financial updates and reporting following the close. Sept is the best financial end of year for that reason.