I beg your pardon. My memory failed me regarding the definitions of "week" versus "work week". "Week" = 7 days starting with Monday. "Work Week = 5 days starting with Monday. I retested and i get 3/19-2018 through 3/25-2018 and 3/19-2018 through 3/23-2018 respectively.

This "starting with Monday" behavior stems from how SQL (at least MS-SQL) works. It numbers days 0 through 6, with 0 being Monday.

I guess someone, somewhere, somtime, thought "weekend" really meant the last two days of the week, else it would have been called week-bookends! :-)

You can see this by running this SQL
SELECT DATEADD(d,-1,GETDATE()) 'Yesterday'
----First Day of Current Week
SELECT DATEADD(wk,DATEDIFF(wk,0,GETDATE()),0) 'First Day of Current Week'
----Last Day of Current Week
SELECT DATEADD(wk,DATEDIFF(wk,0,GETDATE()),6) 'Last Day of Current Week'
----First Day of Last Week
SELECT DATEADD(wk,DATEDIFF(wk,7,GETDATE()),0) 'First Day of Last Week'
----Last Day of Last Week
SELECT DATEADD(wk,DATEDIFF(wk,7,GETDATE()),6) 'Last Day of Last Week'
----First Day of Current Month
SELECT DATEADD(mm,DATEDIFF(mm,0,GETDATE()),0) 'First Day of Current Month'
----Last Day of Current Month
SELECT DATEADD(ms,- 3,DATEADD(mm,0,DATEADD(mm,DATEDIFF(mm,0,GETDATE())+1,0))) 'Last Day of Current Month'
----First Day of Last Month
SELECT DATEADD(mm,-1,DATEADD(mm,DATEDIFF(mm,0,GETDATE()),0)) 'First Day of Last Month'
----Last Day of Last Month
SELECT DATEADD(ms,-3,DATEADD(mm,0,DATEADD(mm,DATEDIFF(mm,0,GETDATE()),0))) 'Last Day of Last Month'
----First Day of Current Year
SELECT DATEADD(yy,DATEDIFF(yy,0,GETDATE()),0) 'First Day of Current Year'
----Last Day of Current Year
SELECT DATEADD(ms,-3,DATEADD(yy,0,DATEADD(yy,DATEDIFF(yy,0,GETDATE())+1,0))) 'Last Day of Current Year'
----First Day of Last Year
SELECT DATEADD(yy,-1,DATEADD(yy,DATEDIFF(yy,0,GETDATE()),0)) 'First Day of Last Year'
----Last Day of Last Year
SELECT DATEADD(ms,-3,DATEADD(yy,0,DATEADD(yy,DATEDIFF(yy,0,GETDATE()),0))) 'Last Day of Last Year' 
SELECT DATEADD(yy,-1,(DATEADD(mm,0,DATEADD(mm,DATEDIFF(mm,0,GETDATE()),0)))) 'First day of current month in previous year'
SELECT DATEADD(second,-1,DATEADD(mm,0,DATEADD(mm,DATEDIFF(mm,0,GETDATE()),0))) 'Last Day of Last Month'
Cintac for sure won't be changing this "standard" behavior in version 6 of the product. In version 7 I'm told calendar presentations (not talking about date pickers here), will be user-definable regarding showing a U.S. style calendar with Sunday in the first column, versus the rest of the world that shows Monday in the first column.

As to your "where would I do that" question, it depends on what exactly your trying to accomplish. You started asking about passing date values in the URL to report filters, then about defaulting report filter values, then switched to view parameters, so I've lost track....

I guess if your ultimate goal is to have a report automatically run for some date range interval, and that interval needs to be dynamic based on today's date, then you'd use some sort of function like the ones in my SQL example above, and put it in the report's fixed condition, or in the where clause of whatever view is being used as the data source for the report.

As far as providing end-users with a choice at runtime and defaulting that choice to something not supported out-of-the-box, it can be rather more involved. You can use only very simplistic expressions to define a default value to a view parameter typed as a date. But report combo-type filter values can come from any column from another report.

I recall building for your company some years ago a custom drop-down list for a payroll period filter showing the date ranges in all the periods. Those periods were a dynamically moving window for something like 6 months. The drop-down values were coming from a list report based on some SQL view that was using date functions. Maybe you could find that report in your system and look at how it works.