Sum Per Month

put this and drag right

=SUMPRODUCT((MONTH($B$3:$KV$3)=MONTH(B$1))*($B$4:$KV$4))

see attached.
Sum-Per-Month.xlsx

correction to my formula above

=SUMPRODUCT((MONTH($B$3:$KV$3)=MONTH(B$1))*((YEAR($B$3:$KV$3)=YEAR(B$1))*($B$4:$KV$4)))
Sum-Per-Month.xlsx

Thank you.  Rgonzo's accounted for the years which is what I was after.  August 2016 and August 2017 were the same answer under Professor's.  I might have not made that so clear though.

glad Rgonzo's solution worked for you. i did not notice at first glace your data had multiple years, so i posted a modified formula which is =SUMPRODUCT((MONTH($B$3:$KV$3)=MONTH(B$1))*((YEAR($B$3:$KV$3)=YEAR(B$1))*($B$4:$KV$4))) which works too.

Thank you for your follow up.  Much appreciated

you are welcome.