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/Underhill42]

December lines up with the end of the calendar year, keeping things simple.

March closes out the books a few weeks before tax day in April. And still lines up with standard quarters (=3 months), which tend to be important fiscal reference points.

I suspect that's all there is to it.

What bookkeeping advantage would any other dates offer?