Q:

SSRS Prior Month, MTD Same period as current month -1 Day

If today is 5/2 I want to see last month's data for 4/1.
For tomorrow on 5/3 I want to see last month's data for 4/2, etc
I want to modify the code to not show me today's value for same time last month, I want the day before today.
I got this far from another question asked here. 
SSRS Prior Month, MTD Same period as current month
 =sum(IIf(Year(Fields!AppDateActualDate.Value) = Year(DateAdd("d",-1, Now).AddMonths(-3)) And Month(Fields!AppDateActualDate.Value) = Month(DateAdd("d",-1, Now).AddMonths(-3)) AND DatePart("d", Fields!AppDateActualDate.Value) <= Dateadd("d",-1, Now), Fields!Application_Count.Value, Nothing), )

A:

I'd create a new data set (assuming you are using SQL  as your data source) and create a default date you are after.. simple as this:
select dateadd(day,-1, dateadd(month,-1, convert(date,getdate()))) as default_date

Then use this date as your parameter for your report. 
That is the start date.. not clear as to what you want your end date to be..

