As I understand it, you want the user to select a month and data to be returned for that month. I was suggesting the following:
Create a month parameter {?month} which is of number datatype. For the value, enter the numbers 1 to 12, and in the description area, enter the corresponding monthnames. Then choose to display the monthname only for the parameter list of values.
I would also create a year parameter, again of number value, so that data from multiple years is not combined. Then in your record selection area, you can use:
month({table.date}) = {?month} and
year({table.date}) = {?year}
To make the report faster, you could create the following two SQL expressions in the field explorer->SQL expression->new:
[%month:]
{fn month(table.`date`)}
[%year:]
{fn year(table.`date`)}
Then you could change your record selection formula to:
{%month} = {?month} and
{%year} = {?year}
...but either record selection approach will work.
-LB