Here is the Form validation formula:
Find days from TODAY() to the first Monday using WEEKDAY(date,return_type), where date is TODAY(), return_type=2. It returns weekdays numbered 1 (Monday) through 7 (Sunday)
Add days (step 1) plus 1 to TODAY() to get the first ...
You can not achieve this using Calculated column.
One option to this is SharePoint Designer as explain below -
Create 3 date columns each for Start Date, Pause Date and End Date.
Create a SharePoint Designer workflow that will trigger when an item is created and when an item is modified.
Write below steps in workflow to compare Stage value and accordingly ...