The only way to have a variable table name would be to use dynamic SQL. You can't use dynamic SQL in a function.

Can you use a proc instead of a function and have the return value changed to an output parameter?

Alternately, you might be able to create a view that selects the required data from all of the tables, and have the function select from the view. If you regularly add tables (which is a really, really bad idea), you could set up a job to alter the view as needed.

A better idea is to change the table structure so that the date is a column instead of part of the table name. That will prevent all kinds of problems.