Rick's formula *will* work for you, *except* in the case where you start
selling in Jan, and there's no blank cell in front of it.

Thanks for catching that....

You can simply insert a new Column B, and leave it blank, and hide it.
Then just adjust Rick's formula to include that extra column:

=IF(COUNTA(C2:N2)0,INDEX(B$1:N$1,SUMPRODUCT(MAX(( B2:M2="")*(C2:N2="Sold")*COLUMN(C2:N2)))-1),"")

....and for the correction.

Rick