SSAS Calculated Field - rolling sum, for a year of days
So the calculated member I came up with is:

sum(
    {ParallelPeriod([Calendar].[Fiscal Calendar].[Date],365,[Calendar].[Fiscal Calendar].CurrentMember):[Calendar].[Fiscal Calendar].CurrentMember},
    [Measures].[Occurrences Received]
)


simple enough, I guess. It could have been the ParallelPeriod for the Year level and 1 though I guess as this is a Fiscal Calendar 365 members at the day level needs to be used.