Previous month:

[font="Courier New"](MONTH((DATEADD("MONTH",-1,GETDATE()))) == 1 ? "January" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 2 ? "February" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 3 ? "March" :

MONTH((DATEADD("MONTH",-1,GETDATE()))) == 4 ? "April" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 5 ? "May" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 6 ? "June" :

MONTH((DATEADD("MONTH",-1,GETDATE()))) == 7 ? "July" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 8 ? "August" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 9 ? "September" :

MONTH((DATEADD("MONTH",-1,GETDATE()))) == 10 ? "October" : MONTH((DATEADD("MONTH",-1,GETDATE()))) == 11 ? "November" :

MONTH((DATEADD("MONTH",-1,GETDATE()))) == 12 ? "December" : "InvalidMonth")

+ (DT_WSTR, 4) DATEPART("YYYY",(DATEADD("MONTH",-1,GETDATE())))

+ "_report.txt"[/font]

I just replaced GETDATE() with (DATEADD("MONTH",-1,GETDATE()))

As far as a reference, I have not found a good one yet.

The documentation on expressions in SSIS is terrible, I tend to Google a lot.