Month Range String Variable

[#1]

Sum({<YearNo={$(=Max(YearNo))},MthName={'Jan','Feb','Mar','Apr','May'}>}Plan_InvoiceSales)

I need some help to either create a variable for    'Jan','Feb','Mar','Apr','May'  which represents (beginning of the year to current month - 1) or use some sort of set analysis functionality to get identical results.

Any help would be appreciated.