what tax year end should xyz use and which test or rule requires this year end 421467

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XYZ, LLC has several individual and corporate members. Abe and Joe, individuals with 4/30 year ends, each have a 23% profits and capital interest. RST, Inc., a corporation with a 6/30 year end, owns a 4% profits and capital interest while DEF, Inc., a corporation with an 8/30 year end, owns a 4.9% profits and capital interest. Finally, thirty other calendar year end individual partners (each with less than a 2% profits and capital interest) own the remaining 45% of the profits and capital interests in XYZ. What tax year end should XYZ use and which test or rule requires this year end?