Please follow below steps.
1. Create a parameter (say 'Select Date') with date data type. Select list as type of Parameter. Populate the Parameter using a field in data source or manually.
2. Create a calculated field like 'worksheet A filter'. It should look like below
[Your Date Column] < Dateadd('day', -7, [Select Date])
3. Create a calculated field like 'worksheet B filter'. It should look like below
[Your Date Column] = [Select Date]
4. Drop these fields on the filter shelves of respective sheets and you should be set.
5. Select a date from Parameter and your viz should update.
I'm not sure I'm on right track or not, but here is a one idea with filtering. (Not using parameter)
[lookup Date] // On primary Data source
[Last Seven Value] // On 2nd data source
Blending_last_seven_SM_9.3.twbx 43.6 KB