You can use a validation rule to check if the value is changed and if the previous value was ON and pop up a message to let the user know that they cannot toggle it off, once it has been toggled on. Might be easier to check if it was manually set vs system set (by workflow rule) as opposed to checking the dates (11-28th of the month). You can allow the system to untoggle but not users so in your validation rule you would check if the value changed and if it was not done by the system
IsChanged(toggle) && IsManuallySet(TRUE) && GetPreviousValue(toggle)= true