I have two ideas:
1) If I have a variable vDate1, and set it to a Date
will call that variable value (e.g. in a text box) and prints out the correct date, i.e. you can use so called dollar sign expression to evaluate the string (giving a variable name, evaluating to the date).
2) You could maybe also make use of the input fields instead of variables, allowing you to change the values of the fields elements in the front end. You can use this field as forecast parameter input and just link it to your week field 1:1. Using week as dimension should give your the parameter for this week for free then.