You'll only be able to use parameters if the calculation is part of a query (say, one that's being run from SSRS).

Depending on exactly what your parameter is passing the code would look something like this:

SUM({NULL:STRTOMEMBER("[Dim Date 1].[CALENDAR].[full date].&["+CSTR(@full date)+"]"},[Measures].[EVENT])

The above is assuming your parameter would be passing a date in the format of "2013-06-21 00:00:00.000".

I hope that helps.