Ok I'll see if I can explain it!

The report I'm creating is specific to a month within a year. This is identified by a period value taken from a look up table within my model which identifies the month. The look up table contains a description of the period i.e. 'May 2006' and the first day of that month etc.
When I create a prompt page to set the period, this works fine and the selected period identifies the first day of the month value to pass to the report, where it is then used by the filter.
Ideally I want to remove this step and where my filter is defined as
[FirstofMonth] = ?Period?
I want it to be something like
[FirstofMonth] = ?Period? OR [FirstofMonth] = _first_of_month(current_date)
with the default taking precedence where the parameter isn't set. I've tried quite a few different methods (this one, case statements, etc) with no luck so far.