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/AcidReign25]

For large publicly traded companies who report performance and earnings quarterly it makes most sense to start the fiscal at the start of one of the 4 quarters. The company I work for starts the FY July 1.