report calculation issue - previosu month sales

[Gabrielle]

I have a report with data for the present month. This is
got from a query which takes in a date parameter for the
report date.

I would like to use the report date and then find out the
sales on the last day of the previous month when this
report was ran.

Then I would like to check another table for the sales on
that date. I have to basically compare current sale with
previous sale. But since I do not have previous sale in
this report query, how do I do this?

Thanks!