Search/filter contacts by "letter sent" between dates

Search/filter contacts by "letter sent" between dates

I'm new here, trying to use the tool effectively.

Recently I sent letters to a group of customers, and marked each as having completed the task, with the subject "sent letter".

Now I want to make followup calls to each of those customers.  I thought that would be straight-forward, but I cant figure out how to construct a filter or search for:

Give me customers with:
     Task "sent letter" complete
     Task completed between "2/20/2012" and "2/25/2012"

Can you tell me how to do that?