Setting report parameters for the subscription.

I am setting a subscription for the report in my Web Application.
Let#%92s say, I want my report to be delivered on a monthly basis and to contain information for the reporting time period.
My report has 2 parameters: StartDate and EndDate.

The question is: how can I set these parameters in the subscription (they will be different for every reporting period)?
Does anybody know?
Thank you in advance.