There's a special type of sub-letter, one that produces the recipient's assignment calendar and embeds it in the letter. Just like other sub-letters, you create these by clicking on the Edit sub-letters link within the letter template. Then click on Add assignment calendar.
When embedded in a letter template, this sub-letter will generate the recipient's assignment calendar with the rules you specify here. Note that we used relative dates for the First assignment and Last assignment fields - this makes the dates dynamic, in this case creating the calendar for four months.