Every extension can produce its own logics. If we consider Extension -> Qlik dashboard bundle -> "Date picker", then selecting date interval this extension select apropriate values in date field. You can change selection in filter pane, but it can change also by internal selection logics. It is not quite obvious for user.
Because of it I recommend either create data island with only calendar date field and make this field hidden (SET HidePreffix script variable) and use some variables for your expression, or use variables directly (without this extension).
Anyway, variables should be created. For example
//Last date most frequently used variable
SET vMaxDate = "=Date(Max({$} [Calendar date]))";
//This variable help use in calculations last weekends
SET vMaxWeekendInterval = "='>='&Date(Max({$<[Calendar weekday]*={'Saturday'}>} [Calendar date]))&'<='&Date(Max({$<[Calendar weekday]*={'Saturday','Sunday'}>} [Calendar date]))";
...
Then you can use it in measures like this (try to make you own logics, in this example I calculate Amount in last weekend):
Sum( {$<[Date]={"$(vMaxWeekendInterval)"}>} [Amount])
If you want to choose days of week, use standart filterpane or extension SimpleFieldSelect (https://developer.qlik.com/garden/598c6b041cfdc8bde4c49def) on island field
Then create variable
SET vWeekDays = "=Concat({$} Distinct Chr(39)&[Calendar weekday]&Chr(39), ',')";
Then use it in your reports
Sum( {$<[WeekDate]*={$(vWeekDays)}>} [Amount])
Or you can choose for selections field in your model, but you should be very careful applying selections
It would be greate if you can divide user selection and reports filter. It would be easy to create and support. Every your variable can be configured in script and you can make very complicated logics.
But if not you can use special extensions (SimpleFieldSelect can also do it) for applying selection or make your own based on API https://help.qlik.com/en-US/sense-developer/June2018/Subsystems/APIs/Content/CapabilityAPIs/FieldAPI...
