If you have an array called myarray that is mo x yearidx (as you state in your question) and you want the average of all mo == j, then you could just write
meanmo(j) = mean(myarray(myarray(:,1)==j),:);
where j is the index of the month you want to average over. Just replace "myarray" with whichever variable you want to average over.
Edit: Kirby pointed out that you're using cell storage instead of matrix, so to use my method you'd need to convert your cell to a matrix first:
myarray = cell2mat(temp);