Set the default value expressions for each date parameter using the standard date/time functions.
You can set today as:
=Today()
Yesterday as:
=DateAdd(DateInterval.Day, -1, Today())
See How to: Add, Change, or Delete Default Values for a Report Parameter for more details.
Edit after comments
Say you have the following parameters:
StartDt and EndDt are just set up as Date/Time:
Set the Default Values expression for each parameter using the expressions above, i.e. =Today()
and =DateAdd(DateInterval.Day, -1, Today())
:
Now, when you load the report for the first time the two parameters are already populated with the Default Values:
Users can then just leave the dates as the defaults as they're already set, or change them as necessary.