This is a request that I submitted circa 13 years ago!  Tasks should have calendars to take into consideration that some tasks can only happen on specific days, such as CAB meetings, for example.

I’ve worked around this limitation by assigning a user to a task and setting their working day to the CAB Meeting day.

Its not great and fails of someone removes the user from the task, so if someone has a working solution to this I’d also like to hear it!

Assuming that you have marked the Start-on-Monday tasks with a custom toggle, how about having a Validation which activates “Every time a record is created or edited” with a check of:

(isNew() || ischanged($StartDate)) && $C_StartOnMonday && toString($StartDate,'ddd') <> 'Mon'

You could instead use a Workflow which changes the Start Date on appropriately marked tasks to the preceding Monday if that’s the outcome you want. The script would be like this and should be run on creation and change to start date:

if ($C_StartOnMonday, DateAddCalendarFixedPeriod($StartDate,'d',case(toString($StartDate,'ddd'),'Sun',-6,'Mon',0,'Tue',-1,'Wed',-2,'Thu',-3,'Fri',-4,-5)), $StartDate)

[Please treat the above as pseudo-script examples for understanding only as they could easily be optimised for better performance (and I’ve not tested them either).]