Calendar 28/28 or 5wks/2wks

only way I see doing it is manually for each month 

Ouch

Insane, recurring calendar exceptions are so common that even free e-mail & calendaring software provides for easy handling of recurring events. 

By the same token other scheduling software also provide for easy handling of recurrent calendar exceptions. It is not uncommon for named resources to have their own calendar exceptions even if they share other exceptions such as holidays.

Maybe if using Excel and transfering the data to any P6 calendar you can do it.  Being so common this issue it is not a joke but a very serious one. It is hard to believe there is no easier way.

In the case you are modeling rain days across several calendars even when not recurrent, adjusting these individually for each calendar can be a nightmare when it can be so easy if editing/deleting the particular calendar exception.