There might be more elegant ways of generating the legends but one approach would be to define before the loop a cell array of legend entries, and then assign them in the loop
So ahead of the loop
legendTxt = legendTxt = {'Jan 19','Feb 19';'March 19', 'April 19';'May 19', 'June 19';...' ...
'July 19', 'August 19';'Sept 19', 'Oct 19';'Nov 19', 'Dec 19'}
and then in the loop
l
legend(legendTxt{ceil(ii/2),:}) % ii goes 1,3,5 ceil(ii/2) gives 1,2,3..6 which is what we want